FLD2
 Document title:
 RIF Framework for Logic Dialects (Second Edition)
 Editors
 Harold Boley, National Research Council, Canada
 Michael Kifer, State University of New York at Stony Brook, USA
 Abstract

This document, developed by the Rule Interchange Format (RIF) Working Group, defines a general RIF Framework for Logic Dialects (RIFFLD). The framework describes mechanisms for specifying the syntax and semantics of logic RIF dialects through a number of generic concepts such as signatures, symbol spaces, semantic structures, and so on. The actual dialects are required to specialize this framework to produce their syntaxes and semantics.
 Status of this Document
 This was converted automatically from the 19 February Editor's Draft.
Copyright © 2008 W3C^{®} (MIT, ERCIM, Keio), All Rights Reserved. W3C liability, trademark and document use rules apply.
Contents
 1 Overview of RIFFLD
 2 Syntactic Framework
 2.1 Syntax of a RIF Dialect as a Specialization of RIFFLD
 2.2 Alphabet
 2.3 Symbol Spaces
 2.4 Terms
 2.5 Schemas for Externally Defined Terms
 2.6 Signatures
 2.7 Presentation Language of a RIF Dialect
 2.8 Wellformed Terms and Formulas
 2.9 Annotations in the Presentation Syntax
 2.10 EBNF Grammar for the Presentation Syntax of RIFFLD
 3 Semantic Framework
 4 XML Serialization Framework
 5 Conformance of RIF Processors with RIF Dialects
 6 References
 7 Appendix: XML Schema for RIFFLD
1 Overview of RIFFLD
The RIF Framework for Logic Dialects (RIFFLD) is a formalism for specifying all logic dialects of RIF, including the RIF Basic Logic Dialect [RIFBLD]. It is a logic in which both syntax and semantics are described through a number of mechanisms that are commonly used for various logic languages, but are rarely brought all together. Amalgamation of several different mechanisms is required because the framework must be broad enough to accommodate several different types of logic languages and because various advanced mechanisms are needed to facilitate translation into a common framework. RIFFLD gives precise definitions to these mechanisms, but allows certain details to vary. The design of RIF envisages that future standard logic dialects will be based on RIFFLD. Therefore, any logic dialect being developed to become a stardard should either be a specialization of FLD or justify its deviations from (or extensions to) FLD.
The framework described in this document is very general and captures most of the popular logic rule languages found in Databases, Logic Programming, and on the Semantic Web. However, it is anticipated that the needs of future dialects might stimulate further evolution of RIFFLD. In particular, future extensions might include a logic rendering of actions as found in production and reactive rule languages. This would support semantic Web services languages such as [SWSLRules] and [WSMLRules].
This document is mostly intended for the designers of future RIF dialects. All logic RIF dialects are required to be derived from RIFFLD by specialization, as explained in Sections Syntax of a RIF Dialect as a Specialization of RIFFLD and Semantics of a RIF Dialect as a Specialization of RIFFLD. In addition to specialization, to lower the barrier of entry for their intended audiences, a dialect designer may choose to specify the syntax and semantics in a direct, but equivalent, way, which does not require familiarity with RIFFLD. For instance, the RIF Basic Logic Dialect [RIFBLD] is specified by specialization from RIFFLD and also directly, without relying on the framework. Thus, the reader who is interested in RIFBLD only can proceed directly to that document.
RIFFLD has the following main components:
 Syntactic framework. This framework defines the mechanisms for specifying the formal presentation syntax of RIF logic dialects by specializing the presentation syntax of the framework. The presentation syntax is used in RIF to define the semantics of the dialects and to illustrate the main ideas with examples. This syntax is not intended to be a concrete syntax for the dialects; it leaves out details such as the delimiters of the various syntactic components, parenthesizing, precedence of operators, and the like. Since RIF is an interchange format, it uses XML as its concrete syntax.
 Semantic framework. The semantic framework describes the mechanisms that are used for specifying the models of RIF logic dialects.
 XML serialization framework. This framework defines the general principles that logic dialects are to use in specifying their concrete XMLbased syntaxes. For each dialect, its concrete XML syntax is a derivative of the dialect's presentation syntax. It can be seen as a serialization of that syntax.
Syntactic framework. The syntactic framework defines six types of RIF terms:
 Constants and variables. These terms are common to most logic languages.
 Positional terms. These terms are commonly used in firstorder logic. RIFFLD defines positional terms in a slightly more general way in order to enable dialects with higherorder syntax, such as HiLog [CKW93].
 Terms with named arguments. These are like positional terms except that each argument of a term is named and the order of the arguments is immaterial. Terms with named arguments generalize the notion of rows in relational tables, where column headings correspond to argument names.
 Frames. A frame term represents an assertion about an object and its properties. These terms correspond to molecules of Flogic [KLW95]. There is syntactic similarity between terms with named arguments and frames, since object properties resemble named arguments. However, the semantics of these terms are different.
 Classification. These terms are used to define the subclass and class membership relationships. Like frames, they are also borrowed from Flogic [KLW95].
 Equality. These terms are used to equate other terms.
 Formula terms. These terms are the ones for which truth values are defined by the RIF semantic framework. Most dialects would treat such terms in a special way and will impose various restrictions on the contexts in which such terms will be allowed to occur. Some advanced dialects, however, will have fewer such restrictions, which will make it possible to reify formulas and manipulate them as objects.
Terms are then used to define several types of RIFBLD formulas. RIF dialects can choose to permit all or some of the aforesaid categories of terms. The syntactic framework also defines the following specialization mechanisms:
 Symbol spaces.
Symbol spaces partition the set of nonlogical symbols that correspond to individual constants, predicates, and functions, and each partition is then given its own semantics. A symbol space has an identifier and a lexical space, which defines the "shape" of the symbols in that symbol space. Some symbol spaces in RIF are used to identify Web entities and their lexical space consists of strings that syntactically look like internationalized resource identifiers [RFC3987], or IRIs (e.g., http://www.w3.org/2007/rif#iri). Other symbol spaces are used to represent the datatypes required by RIF (for example, http://www.w3.org/2001/XMLSchema#integer).
 Signatures.
Signatures determine which terms and formulas are wellformed. It is a generalization of the notion of a sort in classical firstorder logic [Enderton01]. Each nonlogical symbol (and some logical symbols, like =) has an associated signature. A signature defines, in a precise way, the syntactic contexts in which the symbol is allowed to occur.
For instance, the signature associated with a symbol p might allow p to appear in a term of the form f(p), but disallow it to occur in a term like p(a,b). The signature for f, on the other hand, might allow that symbol to appear in f(p) and f(p,q), but disallow f(p,q,r) and f(f). In this way, it is possible to control which symbols are used for predicates and which for functions, where variables can occur, and so on.
Depending on their needs, dialects can decide which symbols have which signatures.
 Restriction.
A dialect might impose further restrictions on the form of a particular kind of terms or formulas.
Semantic framework. This framework defines the notion of a semantic structure (also knows as interpretation in the literature [Enderton01, Mendelson97]). Semantic structures are used to interpret formulas and to define logical entailment. As with the syntax, this framework includes a number of mechanisms that RIF logic dialects can specialize to suit their needs. These mechanisms include:
 Set of truth values. RIFFLD is designed to accommodate dialects that support reasoning with inconsistent and uncertain information. Most of the logics that are designed to deal with these situations are multivalued. Consequently, RIFFLD postulates that there is a set of truth values, TV, which includes the values t (true) and f (false) and possibly others. For example, RIF Basic Logic Dialect [RIFBLD] is twovalued, but other dialects can have additional truth values.
 Semantic structures. Semantic structures determine how the different symbols in the alphabet of a dialect are interpreted and how truth values are assigned to formulas.
 Datatypes. Some symbol spaces that are part of the RIF syntactic framework have fixed interpretations. For instance, symbols in the symbol space http://www.w3.org/2001/XMLSchema#string are always interpreted as sequences of unicode characters, and a ≠ b for any pair of distinct symbols. A symbol space whose symbols have a fixed interpretation in any semantic structure is called a datatype.
 Entailment. This notion is fundamental to logicbased dialects. Given a set of formulas (e.g., facts and rules) G, entailment determines which other formulas necessarily follow from G. Entailment is the main mechanism underlying query answering in databases, logic programming, and the various reasoning tasks in Description Logics.
A set of formulas G logically entails another formula g if for every semantic structure I in some set S, if G is true in I then g is also true in I. Almost all logics define entailment this way. The difference lies in which set S they use. For instance, logics that are based on the classical firstorder predicate calculus, such as most Description Logics, assume that S is the set of all semantic structures. In contrast, most logic programming languages use default negation. Accordingly, the set S contains only the socalled "minimal" Herbrand models of G and, furthermore, only the minimal models of a special kind. See [Shoham87] for a more detailed exposition of this subject.
XML serialization framework. This framework defines the general principles for mapping the presentation syntax of RIFFLD to the concrete XML interchange format. This includes:
 A specification of the XML syntax for RIFFLD, including the associated XML Schema document.
 A specification of a onetoone mapping from the presentation syntax of RIFFLD to its XML syntax. This mapping must map any wellformed group formula of RIFFLD to an XML document that is valid with respect to the aforesaid XML Schema document.
This document is the latest draft of the RIFFLD specification. Each RIF dialect that is derived from RIFFLD will be described in its own document. The first of such dialects, RIF Basic Logic Dialect, is described in [RIFBLD].
2 Syntactic Framework
The next subsection explains how to derive the presentation syntax of a RIF dialect from the presentation syntax of the RIF framework. The actual syntax of the RIF framework is given in subsequent subsections.
2.1 Syntax of a RIF Dialect as a Specialization of RIFFLD
The presentation syntax for a RIF dialect can be obtained from the general syntactic framework of RIF by specializing the following parameters, which are defined later in this document:
 The alphabet of RIFFLD can be restricted (by omitting symbols).
 An assignment of signatures to each constant and variable
symbol.
Signatures determine which terms in the dialect are wellformed and which are not.
The exact way signatures are assigned depends on the dialect. An assignment can be explicit or implicit (for instance, derived from the context in which each symbol is used).
 The choice of the types of terms supported by the dialect.
The RIF logic framework introduces the following types of terms:
 constant
 variable
 positional
 with named arguments
 equality
 frame
 class membership
 subclass
 external
 formulas
A dialect might support all of these terms or just a subset. For instance, some dialects might not support terms with named arguments or frame terms or certain forms of external terms (e.g., external frames). A dialect might even support additional kinds of terms that are not listed above (for instance, typing terms of Flogic [KLW95]).
 The choice of symbol spaces supported by the dialect.
Symbol spaces determine the syntax of the constant symbols that are allowed in the dialect.
 The choice of the formulas supported by the dialect.
RIFFLD offers the following kind of formula terms "out of the box":
 Atomic
 Conjunction
 Disjunction
 Symmetric negation (classical, explicit, or strong)
 Default negation (as in logic programming)
 Rule (as in logic programming as opposed to the classical material implication)
 Quantification (universal and existential)
A dialect might support all of these formulas, it might impose various restrictions, or it might add additional types of formulas. For instance, the formulas allowed in the conclusion and/or premises of implications might be restricted (e.g., [RIFBLD] essentially allows Horn rules only), certain types of quantification might be prohibited (e.g., [RIFBLD] disallows existential quantification in the rule head), symmetric or default negation (or both) might not be allowed (as in RIFBLD), etc. The Core subdialect of RIFBLD disallows equality formulas in the conclusions of the rules.
More interestingly, dialects can introduce additional types of formulas by adding new connectives (e.g., classical implication or biimplication) and quantifiers.
Note that although the presentation syntax of a RIF logic dialect is normative, since semantics is defined in terms of that syntax, the presentation syntax is not intended as a concrete syntax, and conformant systems are not required to implement it.
2.2 Alphabet
Definition (Alphabet). The alphabet of the presentation language of RIFFLD consists of the following disjoint subsets of symbols:
 A countably infinite set of constant symbols Const.
 A countably infinite set of variable symbols Var.
 A countably infinite set of argument names ArgNames.

A finite set of connective symbols, which includes And, Or, Naf, Neg, and :.
Dialects are allowed to extend this repertoire of connectives or restrict it.

A countably infinite set of quantifiers, which includes the symbols Exists_{?X1,...,?Xn} and Forall_{?X1,...,?Xn}, where ?X_{1}, ..., ?X_{n}, n ≥ 1, are variable symbols.
Dialects are allowed to extend this repertoire of quantifiers or restrict it. In the actual presentation syntax, we will be linearizing these symbols and write them as Exists ?X_{1},...,?X_{n} and Forall ?X_{1},...,?X_{n} instead of Exists_{?X1,...,?Xn} and Forall_{?X1,...,?Xn}.
 The symbols =, #, ##, >, External, Dialect, Base, Prefix, and Import.
 The symbols Group and Document.
 Auxiliary symbols (, ), [, ], <, >, and ^^.
Variables are written as Unicode strings preceded by the symbol "?". The argument names in ArgNames are written as Unicode strings that do not start with a "?". The syntax for constant symbols is given in Section Symbol Spaces.
The symbol Naf represents default negation, which is used in rule languages with logic programming and deductive database semantics. Examples of default negation include Clark's negationasfailure [Clark87], the wellfounded negation [GRS91], and stablemodel negation [GL88]. The name of the symbol Naf used here comes from negationasfailure but in RIFFLD this can refer to any kind of default negation.
The symbol Neg represents symmetric negation (as opposed to default negation, which is asymmetric because completely different inference rules are used to derive p and Naf p). Examples of default negation include the classical firstorder negation, explicit negation, and strong negation [APP96].
The symbols =, #, and ## are used in formulas that define equality, class membership, and subclass relationships, respectively. The symbol > is used in terms that have named arguments and in frame terms. The symbol External indicates that an atomic formula or a function term is defined externally (e.g., a builtin), Dialect is a directive used to indicate the dialect of a RIF document (for those dialects that require this), the symbols Base and Prefix enable abridged representations of IRIs, and the symbol Import is an import directive.
Finally, the symbol Document is used for specifying RIFFLD documents and the symbol Group is used to organize RIFFLD formulas into collections. ☐
2.3 Symbol Spaces
Throughout this document, we will be using the following abbreviations:
 xs: stands for the XML Schema URI http://www.w3.org/2001/XMLSchema#
 rdf: stands for http://www.w3.org/1999/02/22rdfsyntaxns#
 pred: stands for http://www.w3.org/2007/rifbuiltinpredicates#
 rif: stands for the URI of RIF, http://www.w3.org/2007/rif#
These and other abbreviations will be used as prefixes in the compact URIlike notation [CURIE], a notation for succinct representation of IRIs [RFC3987]. The precise meaning of this notation in RIF is defined in [RIFDTB].
The set of all constant symbols in a RIF dialect is partitioned into a number of subsets, called symbol spaces, which are used to represent XML Schema datatypes, datatypes defined in other W3C specifications, such as rdf:XMLLiteral, and to distinguish other sets of constants. All constant symbols have a syntax (and sometimes also semantics) imposed by the symbol space to which they belong.
Definition (Symbol space). A symbol space is a named subset of the set of all constants, Const. The semantic aspects of symbol spaces will be described in Section Semantic Framework. Each symbol in Const belongs to exactly one symbol space.
Each symbol space has an associated lexical space and a unique identifier. More precisely,
 The lexical space of a symbol space is a nonempty set of Unicode character strings.
 The identifier of a symbol space is a sequence of Unicode characters that form an absolute IRI [RFC3987].
 Different symbol spaces cannot share the same identifier.
The identifiers for symbol spaces are not themselves constant symbols in RIF. ☐
To simplify the language, we will often use symbol space identifiers to refer to the actual symbol spaces (for instance, we may use "symbol space xs:string" instead of "symbol space identified by xs:string").
To refer to a constant in a particular RIF symbol space, we use the following presentation syntax:
"literal"^^symspace
where literal is called the lexical part of the symbol, and symspace is an identifier of the symbol space. Here literal is a sequence of Unicode characters that must be an element in the lexical space of the symbol space symspace. For instance, "1.2"^^xs:decimal and "1"^^xs:decimal are syntactically valid constants because 1.2 and 1 are members of the lexical space of the XML Schema datatype xs:decimal. On the other hand, "a+2"^^xs:decimal is not a syntactically valid symbol, since a+2 is not part of the lexical space of xs:decimal.
The set of all symbol spaces that partition Const is considered to be part of the logic language of RIFFLD.
RIF requires that all dialects include the symbol spaces listed and described in Section Constants and Symbol Spaces of [RIFDTB] as part of their language. These symbol spaces include constants that belong to several important XML Schema datatypes, certain RDF datatypes, and constant symbols specific to RIF. The latter include the symbol spaces rif:iri and rif:local, which are used to represent internationalized resource identifiers (IRIs [RFC3987]) and constant symbols that are not visible outside of the RIF document in which they occur, respectively. Documents that are exchanged through RIF can use additional symbol spaces.
We will often refer to constant symbols that come from a particular symbol space, X, as Xconstants. For instance the constants in the symbol space rif:iri will be referred to as IRI constants or rif:iri constants and the constants found in the symbol space rif:local as local constants or rif:local constants.
2.4 Terms
The most basic construct of a logic language is a term. RIFFLD supports several kinds of terms: constants, variables, the regular positional terms, plus terms with named arguments, equality, classification terms, and frames. The word "term" will be used to refer to any kind of term.
Definition (Term). A term can have one of the following forms:
 Constants and variables. If t ∈ Const or t ∈ Var then t is a simple term.
 Positional terms. If t and t_{1}, ..., t_{n} are terms then t(t_{1} ... t_{n}) is a positional term.
Positional terms in RIFFLD generalize the regular notion of a term used in firstorder logic. For instance, the above definition allows variables everywhere, as in ?X(?Y ?Z(?V "12"^^xs:integer)), where ?X, ?Y, ?Z, and ?V are variables. Even ?X("abc"^^xs:string ?W)(?Y ?Z(?V "33"^^xs:integer)) is a positional term (as in HiLog [CKW93]).
 Terms with named arguments. A term with named arguments is of the form t(s_{1}>v_{1} ... s_{n}>v_{n}), where t, v_{1}, ..., v_{n} are terms, and s_{1}, ..., s_{n} are (not necessarily distinct) symbols from the set ArgNames.
The term t here represents a predicate or a function; s_{1}, ..., s_{n} represent argument names; and v_{1}, ..., v_{n} represent argument values. Terms with named arguments are like regular positional terms except that the arguments are named and their order is immaterial. Note that a term with no arguments, like f(), is, trivially, both a positional term and a term with named arguments.
For instance, "person"^^xs:string(name>?Y address>?Z), ?X("123"^^xs:integer ?W)(arg>?Y arg2>?Z(?V)), and "Closure"^^rif:local(relation>"http://example.com/Flight"^^rif:iri)(from>?X to>?Y) are terms with named arguments. The second of these terms has a positional term ?X(abc,?W), which occurs in the position of a function, and the third term's function is represented by a named arguments term.
 Equality terms. An equality term has the form t = s, where t and s are terms.
 Classification terms. There are two kinds of classification terms: class membership terms (or just membership terms) and subclass terms.
 t#s is a membership term if t and s are terms.
 t##s is a subclass term if t and s are terms.
Classification terms are used to describe class hierarchies.
 Frame terms. t[p_{1}>v_{1} ... p_{n}>v_{n}] is a frame term (or simply a frame) if t, p_{1}, ..., p_{n}, v_{1}, ..., v_{n}, n ≥ 0, are terms.
Frame terms are used to describe properties of objects. As in the case of the terms with named arguments, the order of the properties p_{i}>v_{i} in a frame is immaterial.

Externally defined terms. If t is a constant, positional term, a term with named arguments, or a frame term then External(t) is an externally defined term.
Such terms are used for representing builtin functions and predicates as well as "procedurally attached" terms or predicates, which might exist in various rulebased systems, but are not specified by RIF.
This syntax enables very flexible representations for externally defined information sources: not only predicates and functions, but also frames can be used. In this way, external sources can be modeled as frames in an objectoriented way. For instance, External("http://example.com/acme"^^rif:iri["http://example.com/mycompany/president"^^rif:iri(?Year) > ?Pres]) could be a representation for an external method "http://example.com/mycompany/president"^^rif:iri in an external object "http://example.com/acme"^^rif:iri. ☐

Formula term. If S is a connective or a quantifier symbol and t_{1}, ..., t_{n} are terms then S(t_{1} ... t_{n}) is a formula term.
Formula terms correspond to compound formulas in logic, i.e., formulas that are constructed from atomic formulas by combining them with connectives and quantifiers. For better visual appeal, some connectives (e.g., the rule implication :) may be written in the infix form, but the above prefix form is considered to be canonical.
The above definitions are very general. They make no distinction between constant symbols that represent individuals, predicates, and function symbols. The same symbol can occur in multiple contexts at the same time. For instance, if p, a, and b are symbols then p(p(a) p(a p c)) is a term. Even variables and general terms are allowed to occur in the position of predicates and function symbols, so p(a)(?v(a c) p) is also a term.
Frame, classification, and other terms can be freely nested, as exemplified by p(?X q#r[p(1,2)>s](d>e f>g)). Some language environments, like FLORA2 [FL2], OO jDREW [OOjD], NxBRE [NxBRE], and CycL [CycL] support fairly large (partially overlapping) subsets of RIFFLD terms, but most languages support much smaller subsets. RIF dialects are expected to carve out the appropriate subsets of RIFFLD terms, and the general form of the RIF logic framework allows a considerable degree of freedom.
Observe that the argument names of frame terms, p_{1}, ..., p_{n}, are terms and, as a special case, can be variables. In contrast, terms with named arguments can use only the symbols from ArgNames to represent their argument names. They cannot be constants from Const or variables from Var. The reason for this restriction has to do with the complexity of unification, which is integral part of many inference rules underlying firstorder logic. We are not aware of any rule language where terms with named arguments use anything more general than what is defined here.
Dialects can restrict the contexts in which the various terms are allowed by using the mechanism of signatures. The RIFFLD language associates a signature with each symbol (both constant and variable symbols) and uses signatures to define wellformed terms. Each RIF dialect is expected to select appropriate signatures for the symbols in its alphabet, and only the terms that are wellformed according to the selected signatures are allowed in that particular dialect.
2.5 Schemas for Externally Defined Terms
This section introduces the notion of external schemas, which serve as templates for externally defined terms. These schemas determine which externally defined terms are acceptable in a RIF dialect. Externally defined terms include RIF builtins, which are specified in [RIFDTB], but are more general. They are designed to accommodate the ideas of procedural attachments and querying of external data sources. Because of the need to accommodate many difference possibilities, the RIF logical framework supports a very general notion of an externally defined term. Such a term is not necessarily a function or a predicate  it can be a frame, a classification term, and so on.
Definition (Schema for external term). An external schema has the form (?X_{1} ... ?X_{n}; τ) where
 τ is a term of one of these kinds: constant, positional, namedargument, frame.
 ?X_{1} ... ?X_{n} is a list of all distinct variables that occur in τ
The names of the variables in an external schema are immaterial, but their order is important. For instance, (?X ?Y; ?X[foo>?Y]) and (?V ?W; ?V[foo>?W]) are considered to be indistinguishable, but (?X ?Y; ?X[foo>?Y]) and (?Y ?X; ?X[foo>?Y]) are viewed as different schemas.
A term t is an instance of an external schema (?X_{1} ... ?X_{n}; τ) iff t can be obtained from τ by a simultaneous substitution ?X_{1}/s_{1} ... ?X_{n}/s_{n} of the variables ?X_{1} ... ?X_{n} with terms s_{1} ... s_{n}, respectively. Some of the terms s_{i} can be variables themselves. For example, ?Z[foo>f(a ?P)] is an instance of (?X ?Y; ?X[foo>?Y]) by the substitution ?X/?Z ?Y/f(a ?P). ☐
Observe that a variable cannot be an instance of an external schema, since τ in the above definition cannot be a variable. It will be seen later that this implies that a term of the form External(?X) is not wellformed in RIF.
The intuition behind the notion of an external schema, such as (?X ?Y; ?X["foo"^^xs:string>?Y]) or (?V; "pred:isTime"^^rif:iri(?V)), is that ?X["foo"^^xs:string>?Y] or "pred:isTime"^^rif:iri(?V) are invocation patterns for querying external sources, and instances of those schemas correspond to concrete invocations. Thus, External("http://foo.bar.com"^^rif:iri["foo"^^xs:string>"123"^^xs:integer]) and External("pred:isTime"^^rif:iri("22:33:44"^^xs:time) are examples of invocations of external terms  one querying an external source and another invoking a builtin.
Definition (Coherent set of external schemas).
A set of external schemas is coherent if there is no term, t, that is an instance of two distinct schemas in the set. ☐
The intuition behind this notion is to ensure that any use of an external term is associated with at most one external schema. This assumption is relied upon in the definition of the semantics of externally defined terms. Note that the coherence condition is easy to verify syntactically and that it implies that schemas like (?X ?Y; ?X[foo>?Y]) and (?Y ?X; ?X[foo>?Y]), which differ only in the order of their variables, cannot be in the same coherent set.
It is important to keep in mind that external schemas are not part of the language in RIF, since they do not appear anywhere in RIF expressions. Instead, like signatures, which are defined below, they are best thought of as part of the grammar of the language. In particular, they will be used to determine which external terms, i.e., the terms of the form External(t) are wellformed.
2.6 Signatures
In this section we introduce the concept of a signature, which is a key mechanism that allows RIFFLD to control the context in which the various symbols are allowed to occur. For instance, a symbol f with signature {(term term) => term, (term) => term} can occur in terms like f(a b), f(f(a b) a), f(f(a)), etc., if a and b have signature term. But f is not allowed to appear in the context f(a b a) because there is no =>expression in the signature of f to support such a context.
The above example provides intuition behind the use of signatures in RIFFLD. Much of the development, below, is inspired by [CK95]. It should be kept in mind that signatures are not part of the logic language in RIF, since they do not appear anywhere in RIFFLD formulas. Instead they are part of the grammar: they are used to determine which sequences of tokens are in the language and which are not. The actual way by which signatures are assigned to the symbols of the language may vary from dialect to dialect. In some dialects (for example [RIFBLD]), this assignment is derived from the context in which each symbol occurs and no separate language for signatures is used. Other dialects may choose to assign signatures explicitly. In that case, they would require a concrete language for signatures (which would be separate from the language for specifying the logic formulas of the dialect).
Definition (Signature name). Let SigNames be a nonempty, partiallyordered finite or countably infinite set of symbols, called signature names. Since signatures are not part of the logic language, their names do not have to be disjoint from Const, Var, and ArgNames. We require that this set includes at least the following reserved signature names:
 atomic  used to represent the syntactic context where atomic formulas are allowed to appear.
 formula  represents the context where formulas (atomic or composite) may appear.
 ∞connective the signature for the connectives And and Or, which can take any number of arguments.
 2connective  the signature for the connectives, such as the rule implication connective :, which takes exactly two arguments.
 1connective  the signature for the connectives that take exactly one quantifier. In our case, this signature will be used for the negation connectives and the quantifiers Forall and Exists.
 =  used for representing contexts where equality terms can appear.
 #  a signature name reserved for membership terms.
 ##  a signature reserved for subclass terms.
 >  a signature reserved for frame terms. ☐
Dialects may introduce additional signature names. For instance, RIF Basic Logic Dialect [RIFBLD] introduces one other signature name, individual. The partial order on SigNames is dialectspecific; it is used in the definition of wellformed terms below.
We use the symbol < to represent the partial order on SigNames. Informally, α < β means that terms with signature α can be used wherever terms with signature β are allowed. We will write α ≤ β if either α = β or α < β.
Definition (Signature). A signature has the form η{e_{1}, ..., e_{n}, ...} where η ∈ SigNames is the name of the signature and {e_{1}, ..., e_{n}, ...} is a countable set of arrow expressions. Such a set can thus be infinite, finite, or even empty. In RIFBLD, signatures can have at most one arrow expression. Other dialects (such as HiLog [CKW93], for example) may require polymorphic symbols and thus allow signatures with more than one arrow expression in them.
An arrow expression is defined as follows:
 If κ, κ_{1}, ..., κ_{n} ∈ SigNames, n≥0, are signature names then (κ_{1} ... κ_{n}) ⇒ κ is a positional arrow expression.
For instance, () ⇒ term and (term) ⇒ term are positional arrow expressions, if term is a signature name.
 If κ, κ_{1}, ..., κ_{n} ∈ SigNames, n≥0, are signature names and p_{1}, ..., p_{n} ∈ ArgNames are argument names then (p_{1}>κ_{1} ... p_{n}>κ_{n}) => κ is an arrow expression with named arguments.
For instance, (arg1>term arg2>term) => term is an arrow signature expression with named arguments. The order of the arguments in arrow expressions with named arguments is immaterial, so any permutation of arguments yields the same expression. ☐
RIF dialects are always associated with sets of coherent signatures, defined next. The overall idea is that a coherent set of signatures must include all the predefined signatures (such as signatures for equality and classification terms) and the signatures included in a coherent set should not conflict with each other. For instance, two different signatures should not have identical names and if one signature is said to extend another then the arrow expressions of the supersignature should be included among the arrow expressions of the subsignature (a kind of an arrow expression "inheritance").
Definition (Coherent signature set). A set Σ of signatures is coherent iff
 Σ contains the special signatures atomic{ } and formula{ }, which represents the context of atomic formulas and more general, composite formulas, respectively. Furthermore, it is required that atomic < formula.
 Σ contains the special signature ∞connective{e_{1}, ..., e_{n}, ...}, where each e_{n} has the form (formula ... formula) ⇒ formula (the lefthand side of this signature is a sequence of n symbols formula). This signature is assigned to the connectives And and Or.
 Σ contains the special signature 2connective{(formula formula) ⇒ formula}. This signature is assigned to the rule implication connective.
 Σ contains the signature 1connective{(formula) ⇒ formula}. This signature is assigned to the negation connectives Naf and Neg, and to the reserved quantifiers of RIFFLD, Exists_{?X1,...,?Xn} and Forall_{?X1,...,?Xn}, for all variable sequences ?X_{1},...,?X_{n} and n ≥ 0.
 Σ contains the signature ={e_{1}, ..., e_{n}, ...} for the equality symbol.
All arrow expressions e_{i} here have the form (κ κ) ⇒ γ (the arguments in an equation must be compatible) and at least one of these expressions must have the form (κ κ) ⇒ atomic (i.e., equation terms are also atomic formulas). Dialects may further specialize this signature.

Σ contains the signature #{e_{1}, ..., e_{n}...}.
Here all arrow expressions e_{i} are binary (have two arguments) and at least one has the form (κ γ) ⇒ atomic. Dialects may further specialize this signature.
 Σ contains the signature ##{e_{1}, ..., e_{n}...}.
Here all arrow expressions e_{i} have the form (κ κ) ⇒ γ (the arguments must be compatible) and at least one of these arrow expressions has the form (κ κ) ⇒ atomic. Dialects may further specialize this signature.
 Σ contains the signature >{e_{1}, ..., e_{n}...}.
 Here all arrow expressions e_{i} are ternary (have three arguments) and at least one of them is of the form (κ_{1} κ_{2} κ_{3}) ⇒ atomic. Dialects may further specialize this signature.
 Σ has at most one signature for any given signature name.

Whenever Σ contains a pair of signatures, ηA and κB, such that η<κ then B⊆A.
Here ηA denotes a signature with the name η and the associated set of arrow expressions A; similarly κB is a signature named κ with the set of expressions B. The requirement that B⊆A ensures that symbols that have signature η can be used wherever the symbols with signature κ are allowed. ☐
The requirement that coherent sets of signatures must include the signatures for =, #, >, and so on is just a technicality that simplifies definitions. Some of these signatures may go "unused" in a dialect even though, technically speaking, they must be present in the signature set associated with that dialect. If a dialect disallows equality, classification terms, or frames in its syntax then the corresponding signatures will remain unused. Such restrictions can be imposed by specializing RIFFLD  see Section Syntax of a RIF Dialect as a Specialization of RIFFLD.
An incoherent set of signatures would be one that includes signatures mysig{() ⇒ atomic} and mysig{(atomic) ⇒ atomic} because it has two different signatures with the same name. Likewise, if this set contains mysig_{1}{() ⇒ atomic} and mysig_{2}{(atomic) ⇒ atomic} and mysig_{1} < mysig_{1} then it is incoherent because the set of arrow expressions of mysig_{1} does not contain the set of arrow expressions of mysig_{2}.
2.7 Presentation Language of a RIF Dialect
The presentation language of a RIF dialect is a set of wellformed formulas, as defined in the next section. The language is determined by the following parameters (see Syntax of a RIF Dialect as a Specialization of RIFFLD):
 An alphabet.
 A set of symbol spaces.
 An assignment of signatures from a coherent set of signatures to the symbols in Var, Const, connectives, and quantifiers:
Each variable symbol is associated with exactly one signature from a coherent set of signatures. A constant symbol can have one or more signatures, and different symbols can be associated with the same signature. (If variables were allowed to have multiple signatures then wellformed terms would not be closed under substitutions. For instance, a term like f(?X,?X) could be wellformed, but f(a,a) could be illformed.)
 Restrictions on the classes of terms allowed in the language of the dialect.
 Restrictions on the classes of formulas allowed in the language of the dialect.
 A coherent set of external schemas.
We have already seen how the alphabet and the symbol spaces are used to define RIF terms. The next section shows how signatures and external schemas are used to further specialize this notion to define wellformed RIFFLD terms.
2.8 Wellformed Terms and Formulas
Since signature names uniquely identify signatures in coherent signature sets, we will often refer to signatures simply by their names. For instance, if one of f's signatures is atomic{ }, we may simply say that symbol f has signature atomic.
Definition (Wellformed term).
 A constant or variable symbol with signature η is a wellformed term with signature η.
 A positional term t(t_{1} ... t_{n}), 0≤n, is wellformed and has a signature σ iff
 t is a wellformed term that has a signature that contains an arrow expression of the form (σ_{1} ... σ_{n}) ⇒ σ; and
 Each t_{i} is a wellformed term whose signature is γ_{i} such that γ_{i}, ≤ σ_{i}.
As a special case, when n=0 we obtain that t( ) is a wellformed term with signature σ, if t's signature contains the arrow expression () ⇒ σ.
 A term with named arguments t(p_{1}>t_{1} ... p_{n}>t_{n}), 0≤n, is wellformed and has a signature σ iff
 t is a wellformed term that has a signature that contains an arrow expression with named arguments of the form (p_{1}>σ_{1} ... p_{n}>σ_{n}) ⇒ σ; and
 Each t_{i} is a wellformed term whose signature is γ_{i}, such that γ_{i} ≤ σ_{i}.
As a special case, when n=0 we obtain that t( ) is a wellformed term with signature σ, if t's signature contains the arrow expression () ⇒ σ.

An equality term of the form t_{1}=t_{2} is wellformed and has a signature κ iff
 The signature = has an arrow expression (σ σ) ⇒ κ
 t_{i} and t_{2} are wellformed terms with signatures γ_{1} and γ_{2}, respectively, such that γ_{i} ≤ σ, i=1,2.
 A membership term of the form t_{1}#t_{2} is wellformed and has a signature κ iff
 The signature # has an arrow expression (σ_{1} σ_{2}) ⇒ κ
 t_{1} and t_{2} are wellformed terms with signatures γ_{1} and γ_{2}, respectively, such that γ_{i} ≤ σ_{i}, i=1,2.
 A subclass term of the form t_{1}##t_{2} is wellformed and has a signature κ iff
 The signature ## has an arrow expression (σ σ) ⇒ κ
 t_{1} and t_{2} are wellformed terms with signatures γ_{1} and γ_{2}, respectively, such that γ_{i} ≤ σ, i=1,2.
 A frame term of the form t[s_{1}>v_{1} ... s_{n}>v_{n}] is wellformed and has a signature κ iff
 The signature > has arrow expressions (σ σ_{11} σ_{12}) ⇒ κ, ..., (σ σ_{n1} σ_{n2}) ⇒ κ (these n expressions need not be distinct).
 t, s_{j}, and v_{j} are wellformed terms with signatures γ, γ_{j1}, and γ_{j2}, respectively, such that γ ≤ σ and γ_{ji} ≤ σ_{ji}, where j=1,...,n and i=1,2.

An externally defined term, External(t), is wellformed and has signature κ iff
 t is wellformed and has signature κ.

t is an instance of an external schema from a coherent set of external schemas of the language.
Note that, according to the definition of coherent sets of schemas, a term can be an instance of at most one external schema. ☐

A formula term, S(t_{1} ... t_{n}), 0≤n, is wellformed if S is a connective or a quantifier whose signature has an arrow expression (σ_{1} ... σ_{n}) ⇒ formula and each t_{i} is a wellformed term whose signature is ≤ σ_{i}.
In the special case of our reserved connectives and quantifiers, t_{1}, ..., t_{n} must have signatures that are below formula (i.e., ≤ formula). Also, if S is : then n must be equal and if S is Neg, Naf, Forall, or Exists then n must be.
Note that, like constant symbols, wellformed terms can have more than one signature. Also note that, according to the above definition, f() and f are distinct terms.
Definition (Wellformed formula).
A wellformed atomic formula is a wellformed term one of whose signatures is atomic or < atomic.
Note that equality, membership, subclass, and frame terms are atomic formulas, since atomic is one of their signatures.
A wellformed formula is
 A wellformed term whose signature is formula or < formula; or
 A group formula; or
 A document formula.
Group and document formulas are defined below. For clarity, we will also give explicit definitions of conjunctive, disjunctive, rule, and other formulas even though they have already defined as special cases of the definition of wellformed formula terms.
 Atomic: If φ is a wellformed atomic formula then it is also a wellformed formula.
 Conjunction: If φ_{1}, ..., φ_{n}, n ≥ 0, are wellformed formula terms then so is And(φ_{1} ... φ_{n}).
As a special case, And() is allowed and is treated as a tautology, i.e., a formula that is always true.
 Disjunction: If φ_{1}, ..., φ_{n}, n ≥ 0, are wellformed formula terms then so is Or(φ_{1} ... φ_{n}).
As a special case, Or() is treated as a contradiction, i.e., a formula that is always false.
 Symmetric negation: If φ is a wellformed formula term then so is Neg φ.
 Default negation: If φ is a wellformed formula term then so is Naf φ.
 Rule implication: If φ and ψ are wellformed formula terms then so is φ : ψ.
 Quantification: If φ is a wellformed formula term and Forall_{?V1,...,?Vn}
and Exists_{?V1,...,?Vn}
are quantifier symbols then
 Exists ?V_{1} ... ?V_{n}(φ)
 Forall ?V_{1} ... ?V_{n}(φ)
are wellformed formula terms.

Group: If φ_{1}, ..., φ_{n} are wellformed formula terms or Groupformulas then Group(φ_{1} ... φ_{n}) is a wellformed group formula.
Group formulas are intended to represent sets of formulas. Note that some of the φ_{i}'s can themselves be group formulas, which means that groups can be nested.

Document: An expression of the form Document(directive_{1} ... directive_{n} Γ) is a wellformed document formula, if
 Γ is an optional wellformed group formula.

directive_{1}, ..., directive_{n} is an optional sequence of directives. A directive can be a dialect directive, a base directive, a prefix directive, or an import directive.
 A dialect directive has the form Dialect(D), where D is a Unicode string that specifies the name of a dialect. This directive specifies the dialect of a RIF document. Some dialects may require this directive in all of its documents, while others (notably, RIFBLD) may not allow it and instead may entirely rely on other syntax. (Purely syntactic identification may not always be possible for dialects that are syntactically identical but semantically different, such as deductive databases with stable model semantics [GL88] and with wellfounded semantics [GRS91]. These two dialects are examples where the Dialect directive might be necessary.)

A base directive has the form Base(iri), where iri is a unicode string in the form of an IRI.
The Base directive defines a syntactic shortcut for expanding relative IRIs into full IRIs, as described in in Section Constants and Symbol Spaces of [RIFDTB].

A prefix directive has the form Prefix(p v), where p is an alphanumeric string that serves as the prefix name and v is an expansion for p  a string that forms an IRI.
(An alphanumeric string is a sequence of ASCII characters, where each character is a letter, a digit, or an underscore "_", and the first character is a letter.)
Like the Base directive, the Prefix directives define shorthands to allow more concise representation of rif:iri constants. This mechanism is explained in [RIFDTB], Section Constants and Symbol Spaces.

An import directive can have one of these two forms: Import(t) or Import(t p). Here t is a rif:iri constant and p is a term. The constant t indicates the address of another document to be imported and p is called the profile of import.
RIFFLD defines the semantics for the directive Import(t) only. The directive Import(t p) is reserved for RIF dialects, which might use it to import nonRIF logical entities, such as RDF data and OWL ontologies [RIFRDF+OWL]. The profile might specify what kind of entity is being imported and under what semantics (for instance, the various RDF entailment regimes can be specified using different profiles).
A document formula can contain at most one Dialect and at most one Base directive. The Dialect directive, if present, must be first, followed by an optional Base directive, followed by any number of Prefix directives, followed by any number of Import directives.
In the definition of a formula, the component formulas φ, φ_{i}, ψ_{i}, and Γ are said to be subformulas of the respective formulas (conjunction, disjunction, nagation, implication, group, etc.) that are built using these components. ☐
Observe that the restrictions in (1)  (8) above imply that groups and documents cannot be nested inside formula terms and documents cannot be nested inside groups.
Example 1 (Signatures, wellformed terms and formulas).
We illustrate the above definitions with the following examples. In addition to atomic, let there be another signature, term{ }, which is intended here to represent the context of the arguments to positional function or atomic formulas.
Consider the term p(p(a) p(a b c)). If p has the (polymorphic) signature mysig{(term)⇒term, (term term)⇒term, (term term term)⇒term} and a, b, c each has the signature term{ } then p(p(a) p(a b c)) is a wellformed term with signature term{ }. If instead p had the signature mysig2{(term term)⇒term, (term term term)⇒term} then p(p(a) p(a b c)) would not be a wellformed term since then p(a) would not be wellformed (in this case, p would have no arrow expression which allows p to take just one argument).
For a more complex example, let r have the signature mysig3{(term)⇒atomic, (atomic term)⇒term, (term term term)⇒term}. Then r(r(a) r(a b c)) is wellformed. The interesting twist here is that r(a) is an atomic formula that occurs as an argument to a function symbol. However, this is allowed by the arrow expression (atomic term)⇒ term, which is part of r's signature. If r's signature were mysig4{(term)⇒atomic, (atomic term)⇒atomic, (term term term)⇒term} instead, then r(r(a) r(a b c)) would be not only a wellformed term, but also a wellformed atomic formula.
An even more interesting example arises when the righthand side of an arrow expression is something other than term or atomic. For instance, let John, Mary, NewYork, and Boston have signatures term{ }; flight and parent have signature h_{2}{(term term)⇒atomic}; and closure has signature hh_{1}{(h_{2})⇒p_{2}}, where p_{2} is the name of the signature p_{2}{(term term)⇒atomic}. Then flight(NewYork Boston), closure(flight)(NewYork Boston), parent(John Mary), and closure(parent)(John Mary) would be wellformed formulas. Such formulas are allowed in languages like HiLog [CKW93], which support predicate constructors like closure in the above example. ☐
2.9 Annotations in the Presentation Syntax
RIFFLD allows every term and formula (including terms and formulas that occur inside other terms and formulas) to be optionally preceded by an annotation of the form (* id φ *) where id is a rif:iri constant and φ is a RIF formula, which is not a documentformula. Both items inside the annotation are optional. The id part represents the identifier of the term (or formula) to which the annotation is attached and φ is the rest of the annotation. RIFFLD does not impose any restrictions on φ apart from what is stated above. This means that φ may include variables, function symbols, rif:local constants, and so on.
Document formulas with and without annotations will be referred to as RIFFLD documents.
A convention is used to avoid a syntactic ambiguity in the above definition. For instance, in (* id φ *) t[w > v] the annotation can be attributed to the term t or to the entire frame t[w > v]. Similarly, for an annotated HiLoglike term of the form (* id φ *) f(a)(b,c), the annotation can be attributed to the entire term f(a)(b,c) or to just f(a). The convention adopted in RIFFLD is that any annotation is syntactically associated with the largest RIFFLD term or formula that appears to the right of that annotation. Therefore, in our examples the annotation (* id φ *) is considered to be attached to the entire frame t[w > v] and to the entire term f(a)(b,c). Yet, since φ can be a conjunction, some conjuncts can be used to provide metadata targeted to the object part, t, of the frame. For instance, (* And(_foo[meta_for_frame>"this is an annotation for the entire frame"] _bar[meta_for_object>"this is an annotation for t" meta_for_property>"this is an annotation for w"] *) t[w > v]. Generally, the convention associates each annotation to the largest term or formula it precedes.
Example 2 (A RIFFLD document with nested groups and annotations).
We illustrate formulas, including documents and groups, with the following complete example (with apologies to Shakespeare for the imperfect rendering of the intended meaning in logic). For better readability, we use the shortcut notation defined in [RIFDTB]. The example also illustrates attachment of annotations.
Document( Prefix(dc http://http://purl.org/dc/terms/) Prefix(ex http://example.org/ontology#) Prefix(hamlet http://www.shakespeareliterature.com/Hamlet/) (* hamlet:assertions hamlet:assertions[dc:title>"Hamlet" dc:creator>"Shakespeare"] *) Group( Exists ?X (And(?X # ex:RottenThing ex:partof(?X <http://www.denmark.dk>))) Forall ?X (Or(hamlet:tobe(?X) Naf hamlet:tobe(?X))) Forall ?X (And(Exists ?B (And(ex:has(?X ?B) ?B # ex:business)) Exists ?D (And(ex:has(?X ?D) ?D # ex:desire))) : ?X # ex:man) (* hamlet:facts *) Group( hamlet:Yorick # ex:poor hamlet:Hamlet # ex:prince ) ) )
Observe that the above set of formulas has a nested subset with its own annotation, hamlet:facts, which contains only a global IRI. ☐
2.10 EBNF Grammar for the Presentation Syntax of RIFFLD
Until now, to specify the syntax of RIFFLD we relied on "mathematical English," a special form of English for communicating mathematical definitions, examples, etc. We will now specify the syntax using the familiar EBNF notation. The following points about the EBNF notation should be kept in mind:
 The syntax of RIFFLD relies on the signature mechanism and is not contextfree, so EBNF does not capture this syntax precisely. As a result, the EBNF grammar defines a strict superset of RIFFLD (not all formulas that are derivable using the EBNF grammar are wellformed).
 The EBNF syntax is not a concrete syntax: it does not address the details of how constants (defined in [RIFDTB]) and variables are represented, and it is not sufficiently precise about the delimiters and escape symbols. White space is informally used as a delimiter, and is implied in productions that use Kleene star. For instance, TERM* is to be understood as TERM TERM ... TERM, where each ' ' abstracts from one or more blanks, tabs, newlines, etc. This is done intentionally since RIF's presentation syntax is used as a tool for specifying the semantics and for illustration of the main RIF concepts through examples.
 RIF defines a concrete syntax only for exchanging rules, and that syntax is XMLbased, obtained as a refinement and serialization of the EBNF syntax via the presentationsyntaxtoXML mapping for RIFFLD.
In view of the above, the EBNF grammar can be viewed as just an intermediary between the mathematical English and the XML. However, it also gives a succinct overview of the syntax of RIFFLD and as such can be useful for dialect designers and users alike.
Document ::= IRIMETA? 'Document' '(' Dialect? Base? Prefix* Import* Group? ')' Dialect ::= 'Dialect' '(' Name ')' Base ::= 'Base' '(' IRI ')' Prefix ::= 'Prefix' '(' Name IRI ')' Import ::= IRIMETA? 'Import' '(' IRICONST PROFILE? ')' Group ::= IRIMETA? 'Group' '(' (FORMULA  Group)* ')' Implies ::= IRIMETA? FORMULA ':' FORMULA FORMULA ::= IRIMETA? 'And' '(' FORMULA* ')'  IRIMETA? 'Or' '(' FORMULA* ')'  Implies  IRIMETA? 'Exists' Var* '(' FORMULA ')'  IRIMETA? 'Forall' Var* '(' FORMULA ')'  IRIMETA? 'Neg' FORMULA  IRIMETA? 'Naf' FORMULA  FORM FORM ::= IRIMETA? (Var  ATOMIC  'External' '(' ATOMIC ')') ATOMIC ::= Const  Atom  Equal  Member  Subclass  Frame Atom ::= UNITERM UNITERM ::= TERM '(' (TERM*  (Name '>' TERM)*) ')' Equal ::= TERM '=' TERM Member ::= TERM '#' TERM Subclass ::= TERM '##' TERM Frame ::= TERM '[' (TERM '>' TERM)* ']' TERM ::= IRIMETA? (Var  EXPRIC  'External' '(' EXPRIC ')') EXPRIC ::= Const  Expr  Equal  Member  Subclass  Frame Expr ::= UNITERM Const ::= '"' UNICODESTRING '"^^' SYMSPACE  CONSTSHORT PROFILE ::= TERM Name ::= UNICODESTRING Var ::= '?' UNICODESTRING SYMSPACE ::= ANGLEBRACKIRI  CURIE IRIMETA ::= '(*' IRICONST? (Frame  'And' '(' Frame* ')')? '*)'
The RIFFLD presentation syntax does not commit to any particular vocabulary and permits arbitrary Unicode strings in constant symbols, argument names, and variables. Constant symbols can have this form: "UNICODESTRING"^^SYMSPACE, where SYMSPACE is a ANGLEBRACKIRI or CURIE that represents the identifier of the symbol space of the constant, and UNICODESTRING is a Unicode string from the lexical space of that symbol space. ANGLEBRACKIRI and CURIE are defined in Section Shortcuts for Constants in RIF's Presentation Syntax of [RIFDTB]. Constant symbols can also have several shortcut forms, which are represented by the nonterminal CONSTSHORT. These shortcuts are also defined in the same section of [RIFDTB]. One of them is the CURIE shortcut, which is used in the examples in this document. Names are Unicode character sequences. Variables are composed of UNICODESTRING symbols prefixed with a ?sign.
RIFFLD formulas and terms can be prefixed with optional annotations, IRIMETA, for identification and metadata. IRIMETA is represented using (*...*)brackets that contain an optional rif:iri constant as identifier followed by an optional Frame or conjunction of Frames as metadata. An IRICONST is the special case of a Const with the symbol space rif:iri, again permitting the shortcut forms defined in [RIFDTB]. One such specialization is '"' IRI '"^^' 'rif:iri' from the Const production, where IRI is a sequence of Unicode characters that forms an internationalized resource identifier as defined by [RFC3987].
3 Semantic Framework
Recall that the presentation syntax of RIFFLD allows the use of shorthand notation, which is specified via the Prefix and Base directives, and various shortcuts for integers, strings, and rif:local symbols. The semantics, below, is described using the full syntax, i.e., we assume that all shortcuts have already been expanded, as defined in [RIFDTB], Section Constants and Symbol Spaces.
3.1 Semantics of a RIF Dialect as a Specialization of RIFFLD
The RIFFLD semantic framework defines the notions of semantic structures and of models for RIFFLD formulas. The semantics of a dialect is derived from these notions by specializing the following parameters.
 The effect of the syntax.

The syntax of a dialect may limit the kinds of terms that are allowed.
For instance, if a dialect's syntax excludes frames or terms with named arguments then the parts of the semantic structures whose purpose is to interpret those types of terms (I_{frame} and I_{NF} in this case) become redundant.
 The dialect might introduce additonal terms and their interpretation by semantic structures.
 The dialect might introduce additional connectives and quantifiers with their interpretation.

The syntax of a dialect may limit the kinds of terms that are allowed.
 Truth values.
The RIFFLD semantic framework allows formulas to have truth values from an arbitrary partially ordered set of truth values, TV. A concrete dialect must select a concrete partially or totally ordered set of truth values.
 Datatypes.
A datatype is a symbol space whose symbols have a fixed interpretation in any semantic structure. RIFFLD defines a set of core datatypes that each dialect is required to include as part of its syntax and semantics. However, RIFFLD does not limit dialects to just the core types: they can introduce additional datatypes, and each dialect must define the exact set of datatypes that it includes.
 Logical entailment.
Logical entailment in RIFFLD is defined with respect to an unspecified set of intended models. A RIF dialect must define which models are considered to be intended. For instance, one dialect might specify that all models are intended (which leads to classical firstorder entailment), another may consider only the minimal models as intended, while a third one might only use wellfounded or stable models [GRS91, GL88].
These notions are defined in the remainder of this specification.
3.2 Truth Values
Definition (Set of truth values). Each RIF dialect must define the set of truth values, denoted by TV. This set must have a partial order, called the truth order, denoted <_{t}. In some dialects, <_{t} can be a total order. We write a ≤_{t} b if either a <_{t} b or a and b are the same element of TV. In addition,
 TV must be a complete lattice with respect to <_{t}, i.e., the least upper bound (lub_{t}) and the greatest lower bound (glb_{t}) must exist for any subset of TV.
 TV is required to have two distinguished elements, f and t, such that f ≤_{t} elt and elt ≤_{t} t for every elt∈TV.
 TV has an operator of negation, ~: TV → TV, such that
 ~ is idempotent, i.e., applying ~ twice gives the identity mapping.
 ~t = f (and thus ~f = t). ☐
RIF dialects can have additional truth values. For instance, the semantics of some versions of NAF, such as wellfounded negation, requires three truth values: t, f, and u (undefined), where f <_{t} u <_{t} t. Handling of contradictions and uncertainty usually requires at least four truth values: t, u, f, and i (inconsistent). In this case, the truth order is partial: f <_{t} u <_{t} t and f <_{t} i <_{t} t.
3.3 Primitive Datatypes
Definition (Primitive datatype). A primitive datatype (or just a datatype, for short) is a symbol space that has
 an associated set, called the value space, and
 a mapping from the lexical space of the symbol space to the value space, called lexicaltovaluespace mapping. ☐
Semantic structures are always defined with respect to a particular set of datatypes, denoted by DTS. In a concrete dialect, DTS always includes the datatypes supported by that dialect. All RIF dialects must support the primitive datatypes that are listed in Section Datatypes of [RIFDTB]. Their value spaces and the lexicaltovaluespace mappings fot these datatypes are described in the same section.
Although the lexical and the value spaces might sometimes look similar, one should not confuse them. Lexical spaces define the syntax of the constant symbols in the RIF language. Value spaces define the meaning of the constants. The lexical and the value spaces are often not even isomorphic. For example, 1.2^^xs:decimal and 1.20^^xs:decimal are two legal  and distinct  constants in RIF because 1.2 and 1.20 belong to the lexical space of xs:decimal. However, these two constants are interpreted by the same element of the value space of the xs:decimal type. Therefore, 1.2^^xs:decimal = 1.20^^xs:decimal is a RIF tautology. Likewise, RIF semantics for datatypes implies certain inequalities. For instance, abc^^xs:string ≠ abcd^^xs:string is a tautology, since the lexicaltovaluespace mapping of the xs:string type maps these two constants into distinct elements in the value space of xs:string.
3.4 Semantic Structures
The central step in specifying a modeltheoretic semantics for a logicbased language is defining the notion of a semantic structure. Semantic structures are used to assign truth values to RIFFLD formulas.
Definition (Semantic structure). A semantic structure, I, is a tuple of the form <TV, DTS, D, I_{C}, I_{V}, I_{F}, I_{frame}, I_{NF}, I_{sub}, I_{isa}, I_{=}, I_{external}, I_{connective}, I_{truth}>. Here D is a nonempty set of elements called the domain of I. We will continue to use Const to refer to the set of all constant symbols and Var to refer to the set of all variable symbols. TV denotes the set of truth values that the semantic structure uses and DTS is a set of identifiers for primitive datatypes.
The other components of I are total mappings defined as follows:
 I_{C} maps Const to elements of D.
This mapping interprets constant symbols.
 I_{V} maps Var to elements of D.
This mapping interprets variable symbols.
 I_{F} maps D to functions D* → D (here D* is a set of all sequences of any finite length over the domain D)
This mapping interprets positional terms.
 I_{NF} interprets terms with named arguments. It is a total mapping from D to the set of total functions of the form SetOfFiniteBags(ArgNames × D) → D.
This is analogous to the interpretation of positional terms with two differences:
 Each pair <s,v> ∈ ArgNames × D represents an argument/value pair instead of just a value in the case of a positional term.
 The argument to a term with named arguments is a finite bag of argument/value pairs rather than a finite ordered sequence of simple elements.
 Bags are used here because the order of the argument/value pairs in a term with named arguments is immaterial and the pairs may repeat: p(a>b a>b). (However, p(a>b a>b) is not equivalent to p(a>b), as we shall see later.)
To see why such repetition can occur, note that argument names may repeat: p(a>b a>c). This can be understood as treating a as a bagvalued argument. Identical argument/value pairs can then arise as a result of a substitution. For instance, p(a>?A a>?B) becomes p(a>b a>b) if the variables ?A and ?B are both instantiated with the symbol b.
 I_{frame} is a total mapping from D to total functions of the form SetOfFiniteBags(D × D) → D.
This mapping interprets frame terms. An argument, d ∈ D, to I_{frame} represents an object and a finite bag {<a1,v1>, ..., <ak,vk>} represents a bag (multiset) of attributevalue pairs for d. We will see shortly how I_{frame} is used to determine the truth valuation of frame terms.
Bags are employed here because the order of the attribute/value pairs in a frame is immaterial and the pairs may repeat. For instance, o[a>b a>b]. Such repetitions arise naturally when variables are instantiated with constants. For instance, o[?A>?B ?C>?D] becomes o[a>b a>b] if variables ?A and ?C are instantiated with the symbol a and ?B, ?D with b. (We shall see later that o[a>b a>b] is equivalent to o[a>b].)
 I_{sub} gives meaning to the subclass relationship. It is a total function D × D → D.
The operator ## is required to be transitive, i.e., c1 ## c2 and c2 ## c3 must imply c1 ## c3. This is ensured by a restriction in Section Interpretation of Formulas.
 I_{isa} gives meaning to class membership. It is a total function D × D → D.
The relationships # and ## are required to have the usual property that all members of a subclass are also members of the superclass, i.e., o # cl and cl ## scl must imply o # scl. This is ensured by a restriction in Section Interpretation of Formulas.
 I_{=} is a total function D × D → D.
It gives meaning to the equality operator.

I_{truth} is a total mapping D → TV.
It is used to define truth valuation for formulas.

I_{external} is a mapping from the coherent set of schemas for externally defined functions to total functions D* → D. For each external schema σ = (?X_{1} ... ?X_{n}; τ) in the coherent set of such schemas associated with the language, I_{external}(σ) is a function of the form D^{n} → D.
For every external schema, σ, associated with the language, I_{external}(σ) is assumed to be specified externally in some document (hence the name external schema). In particular, if σ is a schema of a RIF builtin predicate or function, I_{external}(σ) is specified in [RIFDTB] so that:
 If σ is a schema of a builtin function then I_{external}(σ) must be the function defined in the aforesaid document.
 If σ is a schema of a builtin predicate then I_{truth} ο (I_{external}(σ)) (the composition of I_{truth} and I_{external}(σ), a truthvalued function) must be as specified in [RIFDTB].

I_{connective} is a mapping that assigns every connective and quantifier a function D* → D.
Further constraints on the interaction of this function with I_{truth} will be imposed in order to ensure the intended semantics for each connective and quantifier.
For convenience, we also define the following mapping I on wellformed terms:
 I(k) = I_{C}(k), if k is a symbol in Const
 I(?v) = I_{V}(?v), if ?v is a variable in Var
 I(f(t_{1} ... t_{n})) = I_{F}(I(f))(I(t_{1}),...,I(t_{n}))

I(f(s_{1}>v_{1} ... s_{n}>v_{n})) = I_{NF}(I(f))({<s_{1},I(v_{1})>,...,<s_{n},I(v_{n})>})
Here we use {...} to denote a bag of argument/value pairs.

I(o[a_{1}>v_{1} ... a_{k}>v_{k}]) = I_{frame}(I(o))({<I(a_{1}),I(v_{1})>, ..., <I(a_{n}),I(v_{n})>})
Here {...} denotes a bag of attribute/value pairs. Jumping ahead, we note that duplicate elements in such a bag do not affect the value of I_{frame}(I(o))  see Section Interpretation of Nondocument Formulas. For instance, I(o[a>b a>b]) = I(o[a>b]).
 I(c1##c2) = I_{sub}(I(c1), I(c2))
 I(o#c) = I_{isa}(I(o), I(c))
 I(x=y) = I_{=}(I(x), I(y))

I(External(t)) = I_{external}(σ)(I(s_{1}), ..., I(s_{n})), if t is an instance of the external schema σ = (?X_{1} ... ?X_{n}; τ) by substitution ?X_{1}/s_{1} ... ?X_{n}/s_{n}.
Note that, by definition, External(t) is wellformed only if t is an instance of an external schema. Furthermore, by the definition of coherent sets of external schemas, t can be an instance of at most one such schema, so I(External(t)) is welldefined.

If S is a connective or a quantifier and S(t_{1} ... t_{n}) is a wellformed formula term then
I(S(t_{1} ... t_{n})) = I_{connective}(S)(I(t_{1}) ... I(t_{n}))
The effect of signatures. For every signature, sg, supported by a dialect, there is a subset D_{sg} ⊆ D, called the domain of the signature. Terms that have a given signature, sg, must be mapped by I to D_{sg}, and if a term has more than one signature it must be mapped into the intersection of the corresponding signature domains. To ensure this, the following is required:
 If sg < sg' then D_{sg}⊆D_{sg'}.
 If k is a constant that has signature sg then I_{C}(k) ∈ D_{sg}.
 If ?v is a variable that has signature sg then I_{V}(?v) ∈ D_{sg}.
 If sg has an arrow expression of the form (s1 ... sn)⇒s then, for every d∈D_{sg}, I_{F}(d) must map D_{s1}× ... ×D_{sn} to D_{s}.
 If sg has an arrow expression of the form (p1>s1 ... pn>sn)⇒s then, for every d∈D_{sg}, I_{NF}(d) must map the set {<p1,D_{s1}>, ..., <pn,D_{sn}>} to D_{s}.
 If the signature > has arrow expressions (sg,s_{1},r_{1})⇒k, ..., (sg,s_{n},r_{n})⇒k, then, for every d∈D_{sg}, I_{frame}(d) must map {<D_{s1},D_{r1}>, ..., <D_{sn},D_{rn}>} to D_{k}.
 If the signature # has an arrow expression (s r)⇒k then I_{isa} must map D_{s}×D_{r} to D_{k}.
 If the signature ## has an arrow expression (s s)⇒k then I_{sub} must map D_{s}×D_{s} to D_{k}.
 If the signature = has an arrow expression (s s)⇒k then I_{=} must map D_{s}×D_{s} to D_{k}.
The effect of datatypes. The datatype identifiers in DTS impose the following restrictions. If dt ∈ DTS, let LS_{dt} denote the lexical space of dt, VS_{dt} denote its value space, and L_{dt}: LS_{dt} → VS_{dt} the lexicaltovaluespace mapping. Then the following must hold:
 VS_{dt} ⊆ D; and
 For each constant "lit"^^dt such that lit ∈ LS_{dt}, I_{C}("lit"^^dt) = L_{dt}(lit).
That is, I_{C} must map the constants of a datatype dt in accordance with L_{dt}. ☐
RIFFLD does not impose special requirements on I_{C} for constants in the symbol spaces that do not correspond to the identifiers of the primitive datatypes in DTS. Dialects may have such requirements, however. An example of such a restriction could be a requirement that no constant in a particular symbol space (such as rif:local) can be mapped to VS_{dt} of a datatype dt.
3.5 Annotations and the Formal Semantics
RIFFLD annotations are stripped before the mappings that constitue RIFFLD semantic structures are applied. Likewise, they are stripped before applying the truth valuation, TVal_{I}, defined in the next section. Thus, identifiers and metadata have no effect on the formal semantics.
Note that although annotations associated with RIFFLD formulas are ignored by the semantics, they can be extracted by XML tools. Since annotations are represented by frame terms, they can be reasoned with by the rules. The frame terms used to represent metadata can then be fed to other formulas, thus enabling reasoning about metadata.
3.6 Interpretation of Nondocument Formulas
This section defines how a semantic structure, I, determines the truth value TVal_{I}(φ) of a RIFFLD formula, φ, where φ is any formula other than a document formula. Truth valuation of document formulas is defined in the next section.
To this end, we define a mapping, TVal_{I}, from the set of all nondocument formulas to TV. Note that the definition implies that TVal_{I}(φ) is defined only if the set DTS of the datatypes of I includes all the datatypes mentioned in φ.
Definition (Truth valuation). Truth valuation for wellformed formulas in RIFFLD is determined using the following function, denoted TVal_{I}:
 Constants: TVal_{I}(k) = I_{truth}(I(k)), if k ∈ Const.
 Variables: TVal_{I}(?v) = I_{truth}(I(?v)), if ?v ∈ Var.
 Positional atomic formulas: TVal_{I}(r(t_{1} ... t_{n})) = I_{truth}(I(r(t_{1} ... t_{n}))).
 Atomic formulas with named arguments: TVal_{I}(p(s_{1}>v_{1} ... s_{k}>v_{k})) = I_{truth}(I(p(s_{1}> v_{1} ... s_{k}>v_{k}))).
 Equality: TVal_{I}(x = y) = I_{truth}(I(x = y)).
To ensure that equality has precisely the expected properties, it is required that
 I_{truth}(I(x = y)) = t if and only if I(x) = I(y) and that I_{truth}(I(x = y)) = f otherwise.
 Subclass: TVal_{I}(sc ## cl) = I_{truth}(I(sc ## cl)).
To ensure that the operator ## is transitive, i.e., c1 ## c2 and c2 ## c3 imply c1 ## c3, the following is required: For all c1, c2, c3 ∈ D, glb_{t}(TVal_{I}(c1 ## c2), TVal_{I}(c2 ## c3)) ≤_{t} TVal_{I}(c1 ## c3).
 Membership: TVal_{I}(o # cl) = I_{truth}(I(o # cl)).
To ensure that all members of a subclass are also members of the superclass, i.e., o # cl and cl ## scl implies o # scl, the following is required:
 For all o, cl, scl ∈ D, glb_{t}(TVal_{I}(o # cl), TVal_{I}(cl ## scl)) ≤_{t} TVal_{I}(o # scl).
 Frame: TVal_{I}(o[a_{1}>v_{1} ... a_{k}>v_{k}]) = I_{truth}(I(o[a_{1}>v_{1} ... a_{k}>v_{k}])).
Since the bag of attribute/value pairs represents the conjunction of all the pairs, the following is required:
 TVal_{I}(o[a_{1}>v_{1} ... a_{k}>v_{k}]) = glb_{t}(TVal_{I}(o[a_{1}>v_{1}]), ..., TVal_{I}(o[a_{k}>v_{k}])).

Externally defined atomic formula: TVal_{I}(External(t)) = I_{truth}(I_{external}(σ)(I(s_{1}), ..., I(s_{n}))), if t is an atomic formula that is an instance of the external schema σ = (?X_{1} ... ?X_{n}; τ) by substitution ?X_{1}/s_{1} ... ?X_{n}/s_{n}.
Note that, by definition, External(t) is wellformed only if t is an instance of an external schema. Furthermore, by the definition of coherent sets of external schemas, t can be an instance of at most one such schema, so I(External(t)) is welldefined.

Connectives and quantifiers: if S is a connective or a quantifier and S(t_{1} ... t_{n}) is a wellformed formula term then TVal_{I}(S(t_{1} ... t_{n})) =
I_{truth}(I(S(t_{1} ... t_{n}))).
To ensure the intended semantics for the RIFFLD reserved connectives and quantifiers, the following is also required:

Conjunction: TVal_{I}(And(c_{1} ... c_{n})) = glb_{t}(TVal_{I}(c_{1}), ..., TVal_{I}(c_{n})).
The empty conjunction is treated as a tautology, so TVal_{I}(And()) = t.

Disjunction: TVal_{I}(Or(c_{1} ... c_{n})) = lub_{t}(TVal_{I}(c_{1}), ..., TVal_{I}(c_{n})).
The empty disjunction is treated as a contradiction, so TVal_{I}(Or()) = f.
 Negation: TVal_{I}(Neg Neg φ) = TVal_{I}(φ) and TVal_{I}(Naf φ) = ~TVal_{I}(φ).
The symbol ~ here is the idempotent operator of negation on TV introduced in Section Truth Values.
The symmetric negation, Neg, is sufficiently general to capture many different kinds of such negation. For instance, classical negation would, in addition, require TVal_{I}(Neg φ) = ~TVal_{I}(φ); strong negation (analogous to the one in [APP96]) can be characterized by TVal_{I}(Neg φ) ≤_{t} ~TVal_{I}(φ); and explicit negation (analogous to [APP96]) would require no additional constraints.
Note that both classical and default negation are interpreted the same way in any concrete semantic structure. The difference between the two kinds of negation comes into play when logical entailment is defined.
 Quantification:
 TVal_{I}(Exists ?v_{1} ... ?v_{n} (φ)) = lub_{t}(TVal_{I*}(φ)).
 TVal_{I}(Forall ?v_{1} ... ?v_{n} (φ)) = glb_{t}(TVal_{I*}(φ)).
Here lub_{t} (respectively, glb_{t}) is taken over all interpretations I* of the form <TV, DTS, D, I_{C}, I*_{V}, I_{F}, I_{frame}, I_{NF}, I_{sub}, I_{isa}, I_{=}, I_{external}, I_{connective}, I_{truth}>, which are exactly like I, except that the mapping I*_{V}, is used instead of I_{V}. I*_{V} is defined to coincide with I_{V} on all variables except, possibly, on ?v_{1},... ,?v_{n}.
 Rule implication:
 TVal_{I}(head : body)=t, if TVal_{I}(head) ≥_{t} TVal_{I}(body).
 TVal_{I}(head : body)=f otherwise.

Conjunction: TVal_{I}(And(c_{1} ... c_{n})) = glb_{t}(TVal_{I}(c_{1}), ..., TVal_{I}(c_{n})).
 Groups of formulas:
If Γ is a group formula of the form Group(φ_{1} ... φ_{n}) then
 TVal_{I}(Γ) = glb_{t}(TVal_{I}(φ_{1}), ..., TVal_{I}(φ_{n})).
This means that a group of formulas is treated as a conjunction. ☐
Note that rule implications and equality formulas are always twovalued, even if TV has more than two values.
3.7 Interpretation of Documents
Document formulas are interpreted using semantic multistructures.
Definition (Semantic multistructures). A semantic multistructure is a set {I^{Δ1}, ..., I^{Δn}}, n>0, where I^{Δ1}, ..., I^{Δn} are semantic structures adorned with document formulas. These structures must be identical in all respects except that the mappings I_{C}^{Δ1}, ..., I_{C}^{Δn} might differ on the constants in Const that belong to the rif:local symbol space. The above set is allowed to have at most one semantic structure with the same adornment. ☐
Definition (Imported document).
Let Δ be a document formula and Import(t) be one of its import directives, where t is a rif:iri constant that identifies another document formula, Δ'. In this case, we say that Δ' is directly imported into Δ.
A document formula Δ' is said to be imported into Δ if it is either directly imported into Δ or it is imported (directly or not) into another formula, which is directly imported into Δ. ☐
The above definition considers only oneargument import directives, since twoargument directives are expected to be defined on a casebycase basis by other specifications that need to be integrated with RIF.
The notion of semantic multistructures will now be used to define a semantics for RIF documents.
Definition (Truth valuation of document formulas). Let Δ be a document formula and let Δ_{1}, ..., Δ_{k} be all the RIFFLD document formulas that are imported (directly or indirectly, according to the previous definition) into Δ. Let Γ, Γ_{1}, ..., Γ_{k} denote the respective group formulas associated with these documents. If any of these Γ_{i} is missing (which is a possibility, since every part of a document is optional), assume that it is a tautology, such as a = a, so that every TVal function maps such a Γ_{i} to the truth value t. Let I = {I^{Δ}, I^{Δ1}, ..., I^{Δk}, ...} be a semantic multistructure that contains semantic structures adorned with at least the documents Δ, Δ_{1}, ..., Δ_{k}. Then we define:
 TVal_{I}(Δ) = glb_{t}(TVal_{IΔ}(Γ), TVal_{IΔ1}(Γ_{1}), ..., TVal_{IΔk}(Γ_{k})).
Note that this definition considers only those document formulas that are reachable via the oneargument import directives. Twoargument import directives are not covered by RIFFLD. Their semantics is supposed to be defined by other documents, such as [RIFRDF+OWL]. ☐
The above definitions make the intent behind the rif:local constants clear: occurrences of these constants in different documents can be interpreted differently even if they have the same name. Therefore, each document can choose the names for the rif:local constants freely and without regard to the names of such constants used in the imported documents.
Definition (Models).
Let I be a semantic structure or multistructure. We say that
I is a model of a formula, φ, written as I=φ, iff TVal_{I}(φ) is defined and equals t.
Note that for document formulas I must be a multistructure in order for TVal_{I}(φ) to be defined and for nondocument formulas I is an ordinary semantic structure. ☐
3.8 Intended Semantic Structures
The semantics of a set of formulas, Γ, is the set of its intended semantic multistructures. RIFFLD does not specify what these intended multistructures are, leaving this to RIF dialects. Different logic theories may have different criteria for what is considered an intended semantic multistructure.
For the classical firstorder logic, every model is an intended semantic multistructure. For [RIFBLD], which is based on Horn rules, intended multistructures are defined only for sets of rules: an intended semantic multistructure of a RIFBLD set of formulas, Γ, is the unique minimal Herbrand model of Γ. For the dialects in which rule bodies may contain literals negated with the default negation connective Naf, only some of the minimal Herbrand models of a set of rules are intended. Each logic dialect of RIF must define the set of intended semantic multistructures precisely. The two most common such theories are the wellfounded models [GRS91] and stable models [GL88].
The following example illustrates the notion of intended semantic structures. Suppose Γ consists of a single rule formula p : Naf q. If Naf were interpreted as classical negation, then this rule would be simply equivalent to Or(p q), and so it would have two kinds of models: those where p is true and those where q is true. In contrast to firstorder logic, most rulebased systems do not consider p and q symmetrically. Instead, they view the rule p : Naf q as a statement that p must be true if it is not possible to establish the truth of q. Since it is, indeed, impossible to establish the truth of q, such theories would derive p even though it does not logically follow from Or(p q). The logic underlying rulebased systems also assumes that only the minimal Herbrand models are intended (minimality here is with respect to the set of true facts). Furthermore, although our example has two minimal Herbrand models  one where p is true and q is false, and the other where p is false, but q is true, only the first model is considered to be intended.
The above concept of intended models and the corresponding notion of logical entailment with respect to the intended models, defined below, is due to [Shoham87].
3.9 Logical Entailment
We will now define what it means for a set of RIFFLD formulas to entail another RIFFLD formula. This notion is typically used for defining queries to knowledge bases and for other tasks, such as testing subsumption of concepts (e.g., in OWL). We assume that each set of formulas has an associated set of intended semantic structures.
Definition (Logical entailment).
Let Δ be a RIFFLD document formula and φ a nondocument RIFFLD formula. We say that Δ entails φ, written as Δ = φ, if and only if for every intended semantic multistructure I of Δ for which both TVal_{I}(Δ) and TVal_{IΔ}(φ) are defined, it is the case that TVal_{I}(Δ) ≤_{t} TVal_{IΔ}(φ).
As before, I^{Δ} denotes the component of the multistructure I that is adorned with the document Δ.
Dialects may define other notions of entailment as well. For instance, entailment between Δ and φ when both are document formulas or both are nondocument formulas. ☐
This general notion of entailment covers both firstorder logic and the nonmonotonic logics that underlie many rulebased languages [Shoham87].
Note that one consequence of the multidocument semantics is that local constants specified in one document cannot be queried from another document. For instance, if one document, Δ', has the fact "http://example.com/ppp"^^rif:iri("abc"^^rif:local) while another document formula, Δ, imports Δ' and has the rule "http://example.com/qqq"^^rif:iri(?X) : "http://example.com/ppp"^^rif:iri(?X) , then Δ = "http://example.com/qqq"^^rif:iri("abc"^^rif:local) does not hold. This is because the symbol "abc"^^rif:local in Δ' and Δ is treated as different constants by semantic multistructures.
4 XML Serialization Framework
The RIFFLD XML serialization framework defines
 a normative mapping from the RIFFLD presentation syntax to XML (Section Mapping from the RIFFLD Presentation Syntax to the XML Syntax), and
 a normative XML Schema for the XML syntax (Appendix XML Schema for FLD).
As explained in the overview section, the design of RIF envisions that the presentation syntaxes of future logic RIF dialects will be specializations of the presentation syntax of RIFFLD. This means that every wellformed formula in the presentation syntax of a standard logic RIF dialect must also be wellformed in RIFFLD. The goal of the XML serialization framework is to provide a similar yardstick for the RIF XML syntax. This amounts to the requirement that any conformant XML document for a logic RIF dialect must also be a conformant XML document for RIFFLD (conformance is defined below). In terms of the presentationtoXML syntax mappings, this means that each mapping for a logic RIF dialect must be a restriction of the corresponding mapping for RIFFLD. For instance, the mapping from the presentation syntax of RIFBLD to XML in [RIFBLD] is a restriction of the presentationsyntaxtoXML mapping for RIFFLD. In this way, RIFFLD provides a framework for extensibility and mutual compatibility between XML syntaxes of RIF dialects.
Recall that the syntax of RIFFLD is not contextfree and thus cannot be fully captured by EBNF or XML Schema. Still, validity with respect to XML Schema can be a useful test. To reflect this state of affairs, we define two notions of syntactic correctness. The weaker notion checks correctness only with respect to XML Schema, while the stricter notion represents "true" syntactic correctness.
Definition (Valid XML document in a logic dialect). A valid RIFFLD document in the XML syntax is an XML document that is valid with respect to the XML schema in Appendix XML Schema for FLD.
If a dialect, D, specializes RIFFLD then its XML schema is a specialization of the XML schema of RIFFLD. A valid XML document in D is then one that is valid with respect to the XML schema of D. ☐
Definition (Conformant XML document in a logic dialect). A conformant FLD document in the XML syntax is a valid FLD document in the XML syntax that is the image of a wellformed RIFFLD document in the presentation syntax (see Definition Wellformed formula in Section Wellformed Terms and Formulas) under the presentationtoXML syntax mapping χ_{fld} defined in Section Mapping from the RIFFLD Presentation Syntax to the XML Syntax.
If a dialect, D, specializes RIFFLD then an XML document is conformant with respect to D if and only if it is a valid document in D and it is an image of a wellformed document in the presentation syntax of D.
Note that if D requires the directive Dialect(D) as part of its syntax then this implies that any Dconformant document must have this directive. ☐
A roundtripping of a conformant document in a dialect, D, is a semanticspreserving mapping to a document in any language L followed by a semanticspreserving mapping from the Ldocument back to a conformant Ddocument. While semantically equivalent, the original and the roundtripped Ddocuments need not be identical. Metadata should survive roundtripping.
4.1 XML for the RIFFLD Language
RIFFLD uses [XML1.0] for its XML syntax. The XML serialization for RIFFLD is alternating or fully striped [ANF01]. A fully striped serialization views XML documents as objects and divides all XML tags into class descriptors, called type tags, and property descriptors, called role tags [TRT03]. We follow the tradition of using capitalized names for type tags and lowercase names for role tags.
The alluppercase classes in the EBNF of the presentation syntax, such as FORMULA, become XML Schema groups in Appendix XML Schema for FLD. They are not visible in instance markup. The other classes as well as nonterminals and symbols (such as Exists or =) become XML elements with optional attributes, as shown below.
The RIF serialization framework for the syntax of Section EBNF Grammar for the Presentation Syntax of RIFFLD uses the following XML tags.
While there is a RIFFLD element tag for the Import directive and an attribute for the Dialect directive, there are none for the Base and Prefix directives: they are handled as discussed in Section Mapping of the RIFFLD Rule Language.
 Document (document, with optional 'dialect' attribute, containing optional directive and payload roles)  directive (directive role, containing Import)  payload (payload role, containing Group)  Import (importation, containing location and optional profile)  location (location role, containing IRICONST)  profile (profile role, containing PROFILE)  Group (nested collection of sentences)  sentence (sentence role, containing FORMULA or Group)  Forall (quantified formula for 'Forall', containing declare and formula roles)  Exists (quantified formula for 'Exists', containing declare and formula roles)  declare (declare role, containing a Var)  formula (formula role, containing a FORMULA)  Implies (implication, containing if and then roles)  if (antecedent role, containing FORMULA)  then (consequent role, containing FORMULA)  And (conjunction)  Or (disjunction)  Neg (strong negation, containing a formula role)  Naf (default negation, containing a formula role)  Atom (atom formula, positional or with named arguments)  External (external call, containing a content role)  content (content role, containing an Atom, for predicates, or Expr, for functions)  Member (member formula)  Subclass (subclass formula)  Frame (Frame formula)  object (Member/Frame role containing a TERM or an object description)  op (Atom/Expr role for predicates/functions as operations)  args (Atom/Expr positional arguments role, with fixed 'ordered' attribute, containing n TERMs)  instance (Member instance role)  class (Member class role)  sub (Subclass subclass role)  super (Subclass superclass role)  slot (Atom/Expr or Frame slot role, with fixed 'ordered' attribute, containing a Name or TERM followed by a TERM)  Equal (prefix version of term equation '=')  Expr (expression formula, positional or with named arguments)  left (Equal lefthand side role)  right (Equal righthand side role)  Const (individual, function, or predicate symbol, with optional 'type' attribute)  Name (name of named argument)  Var (logic variable)  id (identifier role, containing IRICONST)  meta (meta role, containing metadata as a Frame or Frame conjunction)
The id and meta elements, which are expansions of the IRIMETA element, can occur optionally as the initial children of any Class element.
The XML Schema Definition of RIFFLD is given in Appendix XML Schema for FLD.
The XML syntax for symbol spaces uses the type attribute associated with the XML element Const. For instance, a literal in the xs:dateTime datatype is represented as <Const type="&xs;dateTime">20071123T03:55:4402:30</Const>. RIFFLD also uses the ordered attribute to indicate that the children of args and slot elements are ordered.
Example 3 (Serialization of a nested RIFFLD group with annotations).
This example shows an XML serialization for the formulas in Example 2. For convenience of reference, the original formulas are included at the top. For better readability, we again use the shortcut syntax defined in [RIFDTB].
Presentation syntax: Document( Dialect(FOL) Prefix(dc http://http://purl.org/dc/terms/) Prefix(ex http://example.org/ontology#) Prefix(hamlet http://www.shakespeareliterature.com/Hamlet/) (* hamlet:assertions hamlet:assertions[dc:title>"Hamlet" dc:creator>"Shakespeare"] *) Group( Exists ?X (And(?X # ex:RottenThing ex:partof(?X <http://www.denmark.dk>))) Forall ?X (Or(hamlet:tobe(?X) Naf hamlet:tobe(?X))) Forall ?X (And(Exists ?B (And(ex:has(?X ?B) ?B # ex:business)) Exists ?D (And(ex:has(?X ?D) ?D # ex:desire))) : ?X # ex:man) (* hamlet:facts *) Group( hamlet:Yorick # ex:poor hamlet:Hamlet # ex:prince ) ) ) XML serialization: <!DOCTYPE Document [ <!ENTITY dc "http://purl.org/dc/terms/"> <!ENTITY ex "http://example.org/ontology#"> <!ENTITY hamlet "http://www.shakespeareliterature.com/Hamlet/"> <!ENTITY rif "http://www.w3.org/2007/rif#"> <!ENTITY xs "http://www.w3.org/2001/XMLSchema#"> ]> <Document dialect="FOL"> <payload> <Group> <meta> <Frame> <object> <Const type="&rif;iri">hamlet:assertions</Const> </object> <slot ordered="yes"> <Const type="&rif;iri">&dc;title</Const> <Const type="&xs;string">Hamlet</Const> </slot> <slot ordered="yes"> <Const type="&rif;iri">&dc;creator</Const> <Const type="&xs;string">Shakespeare</Const> </slot> </Frame> </meta> <sentence> <Exists> <declare><Var>X</Var></declare> <formula> <And> <formula> <Member> <instance><Var>X</Var></instance> <class><Const type="&rif;iri">ex:RottenThing</Const></class> </Member> </formula> <formula> <Atom> <op><Const type="&rif;iri">ex:partof</Const></op> <args ordered="yes"> <Var>X</Var> <Const type="&rif;iri">http://www.denmark.dk</Const> </args> </Atom> </formula> </And> </formula> </Exists> </sentence> <sentence> <Forall> <declare><Var>X</Var></declare> <formula> <Or> <formula> <Atom> <op><Const type="&rif;iri">hamlet:tobe</Const></op> <args ordered="yes"><Var>X</Var></args> </Atom> </formula> <formula> <Naf> <formula> <Atom> <op><Const type="&rif;iri">hamlet:tobe</Const></op> <args ordered="yes"><Var>X</Var></args> </Atom> </formula> </Naf> </formula> </Or> </formula> </Forall> </sentence> <sentence> <Forall> <declare><Var>X</Var></declare> <formula> <Implies> <if> <Member> <instance><Var>X</Var></instance> <class><Const type="&rif;iri">ex:man</Const></class> </Member> </if> <then> <And> <formula> <Exists> <declare><Var>B</Var></declare> <formula> <And> <formula> <Atom> <op><Const type="&rif;iri">ex:has</Const></op> <args> <Var>X</Var> <Var>B</Var> </args> </Atom> </formula> <formula> <Member> <instance><Var>B</Var></instance> <class><Const type="&rif;iri">ex:business</Const></class> </Member> </formula> </And> </formula> </Exists> </formula> <formula> <Exists> <declare><Var>D</Var></declare> <formula> <And> <formula> <Atom> <op><Const type="&rif;iri">ex:has</Const></op> <args> <Var>X</Var> <Var>D</Var> </args> </Atom> </formula> <formula> <Member> <instance><Var>D</Var></instance> <class><Const type="&rif;iri">ex:desire</Const></class> </Member> </formula> </And> </formula> </Exists> </formula> </And> </then> </Implies> </formula> </Forall> </sentence> <sentence> <Group> <meta> <Frame> <object> <Const type="&rif;iri">hamlet:facts</Const> </object> </Frame> </meta> <sentence> <Member> <instance><Const type="&rif;iri">hamlet:Yorick</Const></instance> <class><Const type="&rif;iri">ex:poor</Const></class> </Member> </sentence> <sentence> <Member> <instance><Const type="&rif;iri">hamlet:Hamlet</Const></instance> <class><Const type="&rif;iri">ex:prince</Const></class> </Member> </sentence> </Group> </sentence> </Group> </payload> </Document>
4.2 Mapping from the RIFFLD Presentation Syntax to the XML Syntax
This section defines a normative mapping, χ_{fld}, from the presentation syntax of Section EBNF Grammar for the Presentation Syntax of RIFFLD to the XML syntax of RIFFLD. The mapping is given via tables where each row specifies the mapping of a particular syntactic pattern in the presentation syntax. These patterns appear in the first column of the tables and the bolditalic symbols represent metavariables. The second column represents the corresponding XML patterns, which may contain applications of the mapping χ_{fld} to these metavariables. When an expression χ_{fld}(metavar) occurs in an XML pattern in the right column of a translation table, it should be understood as a recursive application of χ_{fld} to the presentation syntax represented by the metavariable. The XML syntax result of such an application is substituted for the expression χ_{fld}(metavar). A sequence of terms containing metavariables with subscripts is indicated by an ellipsis. A metavariable or a wellformed XML subelement is marked as optional by appending a bolditalic question mark, ?, to its right.
4.2.1 Mapping of the Nonannotated RIFFLD Language
The χ_{fld} mapping from the presentation syntax to the XML syntax of the nonannotated RIFFLD Language is given by the table below. Each row indicates a translation χ_{fld}(Presentation) = XML. Since the presentation syntax of RIFFLD is context sensitive, the mapping must differentiate between the terms that occur in the position of the individuals and the terms that occur as atomic formulas. To this end, in the translation table, the positional and named argument terms that occur in the context of atomic formulas are denoted by the expressions of the form pred(...) and the terms that occur as individuals are denoted by expressions of the form func(...). In the table, each metavariable for an (unnamed) positional argument_{i} is assumed to be instantiated to values unequal to the instantiations of named arguments unicodestring_{j} > filler_{j}. Regarding the last but first row, we assume that shortcuts for constants [RIFDTB] have already been expanded to their full form ("..."^^symspace).
Note that while the Import and the Dialect directives are handled by the presentationtoXML syntax mapping, the Prefix and Base directives are not. Instead, these directives should be handled by expanding the associated shortcuts (compact URIs). Namely, a prefix name declared in a Prefix directive is expanded into the associated IRI, while relative IRIs are completed using the IRI declared in the Base directive. The mapping χ_{fld} applies only to such expanded documents. RIFFLD also allows other treatments of Prefix and Base provided that they produce equivalent XML documents. One such treatment is employed in the examples in this document, especially Example 3. It replaces prefix names with definitions of XML entities as follows. Each Prefix declaration becomes an ENTITY declaration [XML1.0] within a DOCTYPE DTD attached to the RIFFLD Document. The Base directive is mapped to the xml:base attribute [XMLBase] in the XML Document tag. Compact URIs of the form prefix:suffix are then mapped to &prefix;suffix.
Presentation Syntax  XML Syntax 

Document( Dialect(name)? Import(loc_{1} prfl_{1}?) . . . Import(loc_{n} prfl_{n}?) group ) 
<Document dialect="name"?> <directive> <Import> <location>χ_{fld}(loc_{1})</location> <profile>χ_{fld}(prfl_{1})</profile>? </Import> </directive> . . . <directive> <Import> <location>χ_{fld}(loc_{n})</location> <profile>χ_{fld}(prfl_{n})</profile>? </Import> </directive> <payload>χ_{fld}(group)</payload> </Document> 
Group( clause_{1} . . . clause_{n} ) 
<Group> <sentence>χ_{fld}(clause_{1})</sentence> . . . <sentence>χ_{fld}(clause_{n})</sentence> </Group> 
Forall variable_{1} . . . variable_{n} ( body ) 
<Forall> <declare>χ_{fld}(variable_{1})</declare> . . . <declare>χ_{fld}(variable_{n})</declare> <formula>χ_{fld}(body)</formula> </Forall> 
conclusion : condition 
<Implies> <if>χ_{fld}(condition)</if> <then>χ_{fld}(conclusion)</then> </Implies> 
And ( conjunct_{1} . . . conjunct_{n} ) 
<And> <formula>χ_{fld}(conjunct_{1})</formula> . . . <formula>χ_{fld}(conjunct_{n})</formula> </And> 
Or ( disjunct_{1} . . . disjunct_{n} ) 
<Or> <formula>χ_{fld}(disjunct_{1})</formula> . . . <formula>χ_{fld}(disjunct_{n})</formula> </Or> 
Neg form 
<Neg> <formula>χ_{fld}(form)</formula> </Neg> 
Naf form 
<Naf> <formula>χ_{fld}(form)</formula> </Naf> 
Exists variable_{1} . . . variable_{n} ( body ) 
<Exists> <declare>χ_{fld}(variable_{1})</declare> . . . <declare>χ_{fld}(variable_{n})</declare> <formula>χ_{fld}(body)</formula> </Exists> 
External ( atomframexpr ) 
<External> <content>χ_{fld}(atomframexpr)</content> </External> 
pred ( argument_{1} . . . argument_{n} ) 
<Atom> <op>χ_{fld}(pred)</op> <args ordered="yes"> χ_{fld}(argument_{1}) . . . χ_{fld}(argument_{n}) </args> </Atom> 
func ( argument_{1} . . . argument_{n} ) 
<Expr> <op>χ_{fld}(func)</op> <args ordered="yes"> χ_{fld}(argument_{1}) . . . χ_{fld}(argument_{n}) </args> </Expr> 
pred ( unicodestring_{1} > filler_{1} . . . unicodestring_{n} > filler_{n} ) 
<Atom> <op>χ_{fld}(pred)</op> <slot ordered="yes"> <Name>unicodestring_{1}</Name> χ_{fld}(filler_{1}) </slot> . . . <slot ordered="yes"> <Name>unicodestring_{n}</Name> χ_{fld}(filler_{n}) </slot> </Atom> 
func ( unicodestring_{1} > filler_{1} . . . unicodestring_{n} > filler_{n} ) 
<Expr> <op>χ_{fld}(func)</op> <slot ordered="yes"> <Name>unicodestring_{1}</Name> χ_{fld}(filler_{1}) </slot> . . . <slot ordered="yes"> <Name>unicodestring_{n}</Name> χ_{fld}(filler_{n}) </slot> </Expr> 
inst [ key_{1} > filler_{1} . . . key_{n} > filler_{n} ] 
<Frame> <object>χ_{fld}(inst)</object> <slot ordered="yes"> χ_{fld}(key_{1}) χ_{fld}(filler_{1}) </slot> . . . <slot ordered="yes"> χ_{fld}(key_{n}) χ_{fld}(filler_{n}) </slot> </Frame> 
inst # class 
<Member> <instance>χ_{fld}(inst)</instance> <class>χ_{fld}(class)</class> </Member> 
sub ## super 
<Subclass> <sub>χ_{fld}(sub)</sub> <super>χ_{fld}(super)</super> </Subclass> 
left = right 
<Equal> <left>χ_{fld}(left)</left> <right>χ_{fld}(right)</right> </Equal> 
"unicodestring"^^space 
<Const type="space">unicodestring</Const> 
?unicodestring 
<Var>unicodestring</Var> 
4.2.2 Mapping of RIFFLD Annotations
The χ_{fld} mapping from RIFFLD annotations in the presentation syntax to the XML syntax is specified by the table below. It extends the translation table of Section Mapping of the Nonannotated RIFFLD Language. The metavariable Typetag in the presentation and XML syntaxes stands for any of the class names And, Or, External, Document, or Group, Quantifier for Exists or Forall, and Negation for Neg or Naf. The dollar sign, $, stands for any of the binary infix operator names #, ##, =, or :, while Binop stands for their respective class names Member, Subclass, Equal, or Implies. The metavariable attr? is used with Typetag to capture the optional dialect attribute (with its value) of Document. Again, each metavariable for an (unnamed) positional argument_{i} is assumed to be instantiated to values unequal to the instantiations of named arguments unicodestring_{j} > filler_{j}.
Presentation Syntax  XML Syntax 

(* iriconst? frameconj? *) Typetag ( e_{1} . . . e_{n} ) 
<Typetag attr?> <id>χ_{fld}(iriconst)</id>? <meta>χ_{fld}(frameconj)</meta>? e_{1}' . . . e_{n}' </Typetag> where attr, e_{1}', . . ., e_{n}' are defined by the equation χ_{fld}(Typetag(e_{1} . . . e_{n})) = <Typetag attr?>e_{1}' . . . e_{n}'</Typetag> 
(* iriconst? frameconj? *) Quantifier variable_{1} . . . variable_{n} ( body ) 
<Quantifier> <id>χ_{fld}(iriconst)</id>? <meta>χ_{fld}(frameconj)</meta>? <declare>χ_{fld}(variable_{1})</declare> . . . <declare>χ_{fld}(variable_{n})</declare> <formula>χ_{fld}(body)</formula> </Quantifier> 
(* iriconst? frameconj? *) Negation e 
<Negation> <id>χ_{fld}(iriconst)</id>? <meta>χ_{fld}(frameconj)</meta>? χ_{fld}(e) </Negation> 
(* iriconst? frameconj? *) pred ( argument_{1} . . . argument_{n} ) 
<Atom> <id>χ_{fld}(iriconst)</id>? <meta>χ_{fld}(frameconj)</meta>? <op>χ_{fld}(pred)</op> <args ordered="yes"> χ_{fld}(argument_{1}) . . . χ_{fld}(argument_{n}) </args> </Atom> 
(* iriconst? frameconj? *) func ( argument_{1} . . . argument_{n} ) 
<Expr> <id>χ_{fld}(iriconst)</id>? <meta>χ_{fld}(frameconj)</meta>? <op>χ_{fld}(func)</op> <args ordered="yes"> χ_{fld}(argument_{1}) . . . χ_{fld}(argument_{n}) </args> </Expr> 
(* iriconst? frameconj? *) pred ( unicodestring_{1} > filler_{1} . . . unicodestring_{n} > filler_{n} ) 
<Atom> <id>χ_{fld}(iriconst)</id>? <meta>χ_{fld}(frameconj)</meta>? <op>χ_{fld}(pred)</op> <slot ordered="yes"> <Name>unicodestring_{1}</Name> χ_{fld}(filler_{1}) </slot> . . . <slot ordered="yes"> <Name>unicodestring_{n}</Name> χ_{fld}(filler_{n}) </slot> </Atom> 
(* iriconst? frameconj? *) func ( unicodestring_{1} > filler_{1} . . . unicodestring_{n} > filler_{n} ) 
<Expr> <id>χ_{fld}(iriconst)</id>? <meta>χ_{fld}(frameconj)</meta>? <op>χ_{fld}(func)</op> <slot ordered="yes"> <Name>unicodestring_{1}</Name> χ_{fld}(filler_{1}) </slot> . . . <slot ordered="yes"> <Name>unicodestring_{n}</Name> χ_{fld}(filler_{n}) </slot> </Expr> 
(* iriconst? frameconj? *) inst [ key_{1} > filler_{1} . . . key_{n} > filler_{n} ] 
<Frame> <id>χ_{fld}(iriconst)</id>? <meta>χ_{fld}(frameconj)</meta>? <object>χ_{fld}(inst)</object> <slot ordered="yes"> χ_{fld}(key_{1}) χ_{fld}(filler_{1}) </slot> . . . <slot ordered="yes"> χ_{fld}(key_{n}) χ_{fld}(filler_{n}) </slot> </Frame> 
(* iriconst? frameconj? *) e_{1} $ e_{2} 
<Binop> <id>χ_{fld}(iriconst)</id>? <meta>χ_{fld}(frameconj)</meta>? e_{1}' e_{2}' </Binop> where Binop, e_{1}', e_{2}' are defined by the equation χ_{fld}(e_{1} $ e_{2}) = <Binop>e_{1}' e_{2}'</Binop> 
(* iriconst? frameconj? *) unicodestring^^symspace 
<Const type="symspace"> <id>χ_{fld}(iriconst)</id>? <meta>χ_{fld}(frameconj)</meta>? unicodestring </Const> 
(* iriconst? frameconj? *) ?unicodestring 
<Var> <id>χ_{fld}(iriconst)</id>? <meta>χ_{fld}(frameconj)</meta>? unicodestring </Var> 
5 Conformance of RIF Processors with RIF Dialects
RIF does not require or expect conformant systems to implement the presentation syntax of a RIF dialect. Instead, conformance is described in terms of semanticspreserving transformations.
Let Τ be a set of datatypes that includes the datatypes specified in [RIFDTB], and suppose Ε is a set of external predicates and functions that includes the builtins listed in [RIFDTB]. Let D be a RIF dialect (e.g., [RIFBLD]). We say that a formula φ is a D_{Τ,Ε} formula iff
 it is a formula in the dialect D,
 all the datatypes used in φ are in Τ, and
 all the externally defined functions and predicates used in φ are in Ε.
A RIF processor is a conformant D_{Τ,Ε} consumer iff it implements a semanticspreserving mapping, μ, from the set of all D_{Τ,Ε} formulas to the language L of the processor.
Formally, this means that for any pair φ, ψ of D_{Τ,Ε} formulas for which φ =_{D} ψ is defined, φ =_{D} ψ iff μ(φ) =_{L} μ(ψ). Here =_{D} denotes the logical entailment in the RIF dialect D and =_{L} is the logical entailment in the language L of the RIF processor.
A RIF processor is a conformant D_{Τ,Ε} producer iff it implements a semanticspreserving mapping, ν, from a subset of the language L of the processor to the set of D_{Τ,Ε} formulas.
Formally, this means that for any pair φ, ψ of formulas in the aforesaid subset of L for which φ =_{L} ψ is defined, φ =_{L} ψ iff ν(φ) =_{D} ν(ψ).
A conformant document in a logic RIF dialect D is one which conforms to all the syntactic constraints of D, including the ones that cannot be checked by an XML Schema validator (see Definition Conformant XML document in a logic dialect).
6 References
6.1 Normative References
 [RDFCONCEPTS]
 Resource Description Framework (RDF): Concepts and Abstract Syntax, Klyne G., Carroll J. (Editors), W3C Recommendation, 10 February 2004, http://www.w3.org/TR/2004/RECrdfconcepts20040210/. Latest version available at http://www.w3.org/TR/rdfconcepts/.
 [RDFSEMANTICS]
 RDF Semantics, Patrick Hayes, Editor, W3C Recommendation, 10 February 2004, http://www.w3.org/TR/2004/RECrdfmt20040210/. Latest version available at http://www.w3.org/TR/rdfmt/.
 [RDFSCHEMA]
 RDF Vocabulary Description Language 1.0: RDF Schema, Brian McBride, Editor, W3C Recommendation 10 February 2004, http://www.w3.org/TR/rdfschema/.
 [RFC3066]
 RFC 3066  Tags for the Identification of Languages, H. Alvestrand, IETF, January 2001. This document is at http://www.isi.edu/innotes/rfc3066.txt.
 [RFC3987]
 RFC 3987  Internationalized Resource Identifiers (IRIs), M. Duerst and M. Suignard, IETF, January 2005. This document is at http://www.ietf.org/rfc/rfc3987.txt.
 [RIFDTB]
 RIF Datatypes and BuiltIns 1.0, Polleres A., Boley H. and Kifer M. (Editors), W3C Rule Interchange Format Working Group Draft. Latest Version available at http://www.w3.org/2005/rules/wiki/DTB.
 [RIFRDF+OWL]
 RIF RDF and OWL Compatibility, de Bruijn, J. (Editor), W3C Rule Interchange Format Working Group Draft. Latest Version available at http://www.w3.org/2005/rules/wiki/SWC.
 [XML1.0]
 Extensible Markup Language (XML) 1.0 (Fourth Edition), W3C Recommendation, World Wide Web Consortium, 16 August 2006, edited in place 29 September 2006. This version is http://www.w3.org/TR/2006/RECxml20060816/.
 [XMLBase]
 XML Base, W3C Recommendation, World Wide Web Consortium, 27 June 2001. This version is http://www.w3.org/TR/2001/RECxmlbase20010627/. The latest version is available at http://www.w3.org/TR/xmlbase/.
 [XMLSCHEMA2]
 XML Schema Part 2: Datatypes, W3C Recommendation, World Wide Web Consortium, 2 May 2001. This version is http://www.w3.org/TR/2001/RECxmlschema220010502/. The latest version is available at http://www.w3.org/TR/xmlschema2/.
6.2 Informational References
 [ANF01]
 Normal Form Conventions for XML Representations of Structured Data, Henry S. Thompson. October 2001.
 [APP96]
 Strong and Explicit Negation in NonMonotonic Reasoning and Logic Programming, J.J. Alferes, L.M. Pereira, and T.C. Przymusinski. Lecture Notes In Computer Science, vol. 1126. Proceedings of the European Workshop on Logics in Artificial Intelligence, 1996.
 [Clark87]
 Negation as failure, K. Clark. Readings in nonmonotonic reasoning, Morgan Kaufmann Publishers, pages 311  325, 1987. (Originally published in 1978.)
 [CK95]
 Sorted HiLog: Sorts in HigherOrder Logic Data Languages, W. Chen, M. Kifer. Sixth Intl. Conference on Database Theory, Prague, Czech Republic, January 1995, Lecture Notes in Computer Science 893, Springer Verlag, pp. 252265.
 [CKW93]
 HiLog: A Foundation for higherorder logic programming, W. Chen, M. Kifer, D.S. Warren. Journal of Logic Programming, vol. 15, no. 3, February 1993, pp. 187230.
 [CURIE]
 CURIE Syntax 1.0: A syntax for expressing Compact URIs, Mark Birbeck, Shane McCarron. W3C Working Draft 2 April 2008. Available at http://www.w3.org/TR/curie/.
 [CycL]
 The Syntax of CycL, Web site. Available at http://www.cyc.com/cycdoc/ref/cyclsyntax.html.
 [Enderton01]
 A Mathematical Introduction to Logic, Second Edition, H. B. Enderton. Academic Press, 2001.
 [FL2]
 FLORA2: An ObjectOriented Knowledge Base Language, M. Kifer. Web site. Available at http://flora.sourceforge.net.
 [GL88]
 The Stable Model Semantics for Logic Programming, M. Gelfond and V. Lifschitz. Logic Programming: Proceedings of the Fifth Conference and Symposium, pages 10701080, 1988.
 [GRS91]
 The WellFounded Semantics for General Logic Programs, A. Van Gelder, K.A. Ross, J.S. Schlipf. Journal of ACM, 38:3, pages 620650, 1991.
 [KLW95]
 Logical foundations of objectoriented and framebased languages, M. Kifer, G. Lausen, J. Wu. Journal of ACM, July 1995, pp. 741843.
 [Mendelson97]
 Introduction to Mathematical Logic, Fourth Edition, E. Mendelson. Chapman & Hall, 1997.
 [NxBRE]
 .NET Business Rule Engine, Web site. Available at http://nxbre.wiki.sourceforge.net/.
 [OOjD]
 ObjectOriented jDREW, Web site. Available at http://www.jdrew.org/oojdrew/.
 [RDFSYN04]
 RDF/XML Syntax Specification (Revised), Dave Beckett, Editor, W3C Recommendation, 10 February 2004, http://www.w3.org/TR/2004/RECrdfsyntaxgrammar20040210/. Latest version available at http://www.w3.org/TR/rdfsyntaxgrammar/.
 [RIFBLD]
 RIF Basic Logic Dialect, Boley H. and Kifer M. (Editors), W3C Rule Interchange Format Working Group Draft. Latest Version available at http://www.w3.org/2005/rules/wiki/BLD.
 [Shoham87]
 Nonmonotonic logics: meaning and utility, Y. Shoham. Proc. 10th International Joint Conference on Artificial Intelligence, Morgan Kaufmann, pp. 388393, 1987.
 [SWSLRules]
 Semantic Web Services Language (SWSL), S. Battle, A. Bernstein, H. Boley, B. Grosof, M. Gruninger, R. Hull, M. Kifer, D. Martin, S. McIlraith, D. McGuinness, J. Su, S. Tabet. W3C Member Submission, September 2005. Available at http://www.w3.org/Submission/SWSFSWSL/.
 [TRT03]
 ObjectOriented RuleML: UserLevel Roles, URIGrounded Clauses, and OrderSorted Terms, H. Boley. Springer LNCS 2876, Oct. 2003, pp. 116. Preprint at http://iititi.nrccnrc.gc.ca/publications/nrc46502_e.html.
 [vEK76]
 The semantics of predicate logic as a programming language, M. van Emden and R. Kowalski. Journal of the ACM 23 (1976), 733742.
 [WSMLRules]
 Web Service Modeling Language (WSML), J. de Bruijn, D. Fensel, U. Keller, M. Kifer, H. Lausen, R. Krummenacher, A. Polleres, L. Predoiu. W3C Member Submission, June 2005. Available at http://www.w3.org/Submission/WSML/.
7 Appendix: XML Schema for RIFFLD
The namespace of RIF is http://www.w3.org/2007/rif#.
XML schemas for the RIFFLD language are defined below and are also available here with additional examples. For modularity, we define a Baseline schema and a Skyline schema. Baseline is the schema module that provides the foundation up to FORMULAs without Implies. Skyline provides the full schema by augmenting Baseline with the Implies FORMULA as well as with Group and Document.
7.1 Baseline Schema Module
<?xml version="1.0" encoding="UTF8"?> <xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" xmlns="http://www.w3.org/2007/rif#" targetNamespace="http://www.w3.org/2007/rif#" elementFormDefault="qualified" version="Id: FLDBaseline.xsd, v. 0.96, 20080715, hboley/dhirtle"> <xs:annotation> <xs:documentation> This is the Baseline module of FLD. It is the foundation of the full schema defined through the Skyline module. The Baseline XML schema is based on the following EBNF (compared to the full EBNF of RIFFLD, Group and Document are omitted, and 'Implies' is missing from the production for FORMULA). FORMULA ::= IRIMETA? 'And' '(' FORMULA* ')'  IRIMETA? 'Or' '(' FORMULA* ')'  IRIMETA? 'Exists' Var* '(' FORMULA ')'  IRIMETA? 'Forall' Var* '(' FORMULA ')'  IRIMETA? 'Neg' FORMULA  IRIMETA? 'Naf' FORMULA  FORM FORM ::= IRIMETA? (Var  ATOMIC  'External' '(' ATOMIC ')') ATOMIC ::= Const  Atom  Equal  Member  Subclass  Frame Atom ::= UNITERM UNITERM ::= TERM '(' (TERM*  (Name '>' TERM)*) ')' Equal ::= TERM '=' TERM Member ::= TERM '#' TERM Subclass ::= TERM '##' TERM Frame ::= TERM '[' (TERM '>' TERM)* ']' TERM ::= IRIMETA? (Var  EXPRIC  'External' '(' EXPRIC ')') EXPRIC ::= Const  Expr  Equal  Member  Subclass  Frame Expr ::= UNITERM Const ::= '"' UNICODESTRING '"^^' SYMSPACE  CONSTSHORT Name ::= UNICODESTRING Var ::= '?' UNICODESTRING SYMSPACE ::= ANGLEBRACKIRI  CURIE IRIMETA ::= '(*' IRICONST? (Frame  'And' '(' Frame* ')')? '*)' </xs:documentation> </xs:annotation> <xs:group name="FORMULA"> <! 'Implies' omitted from Baseline schema, allowing its modular use FORMULA ::= IRIMETA? 'And' '(' FORMULA* ')'  IRIMETA? 'Or' '(' FORMULA* ')'  IRIMETA? 'Exists' Var* '(' FORMULA ')'  IRIMETA? 'Forall' Var* '(' FORMULA ')'  IRIMETA? 'Neg' FORMULA  IRIMETA? 'Naf' FORMULA  FORM > <xs:choice> <xs:element ref="And"/> <xs:element ref="Or"/> <xs:element ref="Exists"/> <xs:element ref="Forall"/> <xs:element ref="Neg"/> <xs:element ref="Naf"/> <xs:group ref="FORM"/> </xs:choice> </xs:group> <xs:complexType name="ExternalFORMULA.type"> <! sensitive to FORMULA (Atom  Frame) context> <xs:sequence> <xs:group ref="IRIMETA" minOccurs="0" maxOccurs="1"/> <xs:element name="content" type="contentFORMULA.type"/> </xs:sequence> </xs:complexType> <xs:complexType name="contentFORMULA.type"> <! sensitive to FORMULA (Atom  Frame) context> <xs:sequence> <xs:choice> <xs:element ref="Atom"/> <xs:element ref="Frame"/> </xs:choice> </xs:sequence> </xs:complexType> <xs:element name="And"> <xs:complexType> <xs:sequence> <xs:group ref="IRIMETA" minOccurs="0" maxOccurs="1"/> <xs:element ref="formula" minOccurs="0" maxOccurs="unbounded"/> </xs:sequence> </xs:complexType> </xs:element> <xs:element name="Or"> <xs:complexType> <xs:sequence> <xs:group ref="IRIMETA" minOccurs="0" maxOccurs="1"/> <xs:element ref="formula" minOccurs="0" maxOccurs="unbounded"/> </xs:sequence> </xs:complexType> </xs:element> <xs:element name="Exists"> <xs:complexType> <xs:sequence> <xs:group ref="IRIMETA" minOccurs="0" maxOccurs="1"/> <xs:element ref="declare" minOccurs="0" maxOccurs="unbounded"/> <xs:element ref="formula"/> </xs:sequence> </xs:complexType> </xs:element> <xs:element name="Forall"> <xs:complexType> <xs:sequence> <xs:group ref="IRIMETA" minOccurs="0" maxOccurs="1"/> <xs:element ref="declare" minOccurs="0" maxOccurs="unbounded"/> <xs:element ref="formula"/> </xs:sequence> </xs:complexType> </xs:element> <xs:element name="Neg"> <xs:complexType> <xs:sequence> <xs:group ref="IRIMETA" minOccurs="0" maxOccurs="1"/> <xs:element ref="formula" minOccurs="1" maxOccurs="1"/> </xs:sequence> </xs:complexType> </xs:element> <xs:element name="Naf"> <xs:complexType> <xs:sequence> <xs:group ref="IRIMETA" minOccurs="0" maxOccurs="1"/> <xs:element ref="formula" minOccurs="1" maxOccurs="1"/> </xs:sequence> </xs:complexType> </xs:element> <xs:element name="formula"> <xs:complexType> <xs:sequence> <xs:group ref="FORMULA"/> </xs:sequence> </xs:complexType> </xs:element> <xs:element name="declare"> <xs:complexType> <xs:sequence> <xs:element ref="Var"/> </xs:sequence> </xs:complexType> </xs:element> <xs:group name="FORM"> <! FORM ::= IRIMETA? (Var  ATOMIC  'External' '(' ATOMIC ')') > <xs:choice> <xs:element ref="Var"/> <xs:group ref="ATOMIC"/> <xs:element name="External" type="ExternalFORM.type"/> </xs:choice> </xs:group> <xs:complexType name="ExternalFORM.type"> <! sensitive to FORM (ATOMIC) context> <xs:sequence> <xs:group ref="IRIMETA" minOccurs="0" maxOccurs="1"/> <xs:element name="content" type="contentFORM.type"/> </xs:sequence> </xs:complexType> <xs:complexType name="contentFORM.type"> <! sensitive to FORM (ATOMIC) context> <xs:sequence> <xs:group ref="ATOMIC"/> </xs:sequence> </xs:complexType> <xs:group name="ATOMIC"> <! ATOMIC ::= Const  Atom  Equal  Member  Subclass  Frame > <xs:choice> <xs:element ref="Const"/> <xs:element ref="Atom"/> <xs:element ref="Equal"/> <xs:element ref="Member"/> <xs:element ref="Subclass"/> <xs:element ref="Frame"/> </xs:choice> </xs:group> <xs:element name="Atom"> <! Atom ::= UNITERM > <xs:complexType> <xs:sequence> <xs:group ref="UNITERM"/> </xs:sequence> </xs:complexType> </xs:element> <xs:group name="UNITERM"> <! UNITERM ::= TERM '(' (TERM*  (Name '>' TERM)*) ')' > <xs:sequence> <xs:group ref="IRIMETA" minOccurs="0" maxOccurs="1"/> <xs:element ref="op"/> <xs:choice> <xs:element ref="args" minOccurs="0" maxOccurs="1"/> <xs:element name="slot" type="slotUNITERM.type" minOccurs="0" maxOccurs="unbounded"/> </xs:choice> </xs:sequence> </xs:group> <xs:element name="op"> <xs:complexType> <xs:sequence> <xs:group ref="TERM"/> </xs:sequence> </xs:complexType> </xs:element> <xs:element name="args"> <xs:complexType> <xs:sequence> <xs:group ref="TERM" minOccurs="0" maxOccurs="unbounded"/> </xs:sequence> <xs:attribute name="ordered" type="xs:string" fixed="yes"/> </xs:complexType> </xs:element> <xs:complexType name="slotUNITERM.type"> <! sensitive to UNITERM (Name) context> <xs:sequence> <xs:element ref="Name"/> <xs:group ref="TERM"/> </xs:sequence> <xs:attribute name="ordered" type="xs:string" fixed="yes"/> </xs:complexType> <xs:element name="Equal"> <! Equal ::= TERM '=' TERM > <xs:complexType> <xs:sequence> <xs:group ref="IRIMETA" minOccurs="0" maxOccurs="1"/> <xs:element ref="left"/> <xs:element ref="right"/> </xs:sequence> </xs:complexType> </xs:element> <xs:element name="left"> <xs:complexType> <xs:sequence> <xs:group ref="TERM"/> </xs:sequence> </xs:complexType> </xs:element> <xs:element name="right"> <xs:complexType> <xs:sequence> <xs:group ref="TERM"/> </xs:sequence> </xs:complexType> </xs:element> <xs:element name="Member"> <! Member ::= TERM '#' TERM > <xs:complexType> <xs:sequence> <xs:group ref="IRIMETA" minOccurs="0" maxOccurs="1"/> <xs:element ref="instance"/> <xs:element ref="class"/> </xs:sequence> </xs:complexType> </xs:element> <xs:element name="Subclass"> <! Subclass ::= TERM '##' TERM > <xs:complexType> <xs:sequence> <xs:group ref="IRIMETA" minOccurs="0" maxOccurs="1"/> <xs:element ref="sub"/> <xs:element ref="super"/> </xs:sequence> </xs:complexType> </xs:element> <xs:element name="instance"> <xs:complexType> <xs:sequence> <xs:group ref="TERM"/> </xs:sequence> </xs:complexType> </xs:element> <xs:element name="class"> <xs:complexType> <xs:sequence> <xs:group ref="TERM"/> </xs:sequence> </xs:complexType> </xs:element> <xs:element name="sub"> <xs:complexType> <xs:sequence> <xs:group ref="TERM"/> </xs:sequence> </xs:complexType> </xs:element> <xs:element name="super"> <xs:complexType> <xs:sequence> <xs:group ref="TERM"/> </xs:sequence> </xs:complexType> </xs:element> <xs:element name="Frame"> <! Frame ::= TERM '[' (TERM '>' TERM)* ']' > <xs:complexType> <xs:sequence> <xs:group ref="IRIMETA" minOccurs="0" maxOccurs="1"/> <xs:element ref="object"/> <xs:element name="slot" type="slotFrame.type" minOccurs="0" maxOccurs="unbounded"/> </xs:sequence> </xs:complexType> </xs:element> <xs:element name="object"> <xs:complexType> <xs:sequence> <xs:group ref="TERM"/> </xs:sequence> </xs:complexType> </xs:element> <xs:complexType name="slotFrame.type"> <! sensitive to Frame (TERM) context> <xs:sequence> <xs:group ref="TERM"/> <xs:group ref="TERM"/> </xs:sequence> <xs:attribute name="ordered" type="xs:string" fixed="yes"/> </xs:complexType> <xs:group name="TERM"> <! TERM ::= IRIMETA? (Var  EXPRIC  'External' '(' EXPRIC ')') > <xs:choice> <xs:element ref="Var"/> <xs:group ref="EXPRIC"/> <xs:element name="External" type="ExternalTERM.type"/> </xs:choice> </xs:group> <xs:complexType name="ExternalTERM.type"> <! sensitive to TERM (EXPRIC) context> <xs:sequence> <xs:group ref="IRIMETA" minOccurs="0" maxOccurs="1"/> <xs:element name="content" type="contentTERM.type"/> </xs:sequence> </xs:complexType> <xs:complexType name="contentTERM.type"> <! sensitive to TERM (EXPRIC) context> <xs:sequence> <xs:group ref="EXPRIC"/> </xs:sequence> </xs:complexType> <xs:group name="EXPRIC"> <! EXPRIC ::= Const  Expr  Equal  Member  Subclass  Frame > <xs:choice> <xs:element ref="Const"/> <xs:element ref="Expr"/> <xs:element ref="Equal"/> <xs:element ref="Member"/> <xs:element ref="Subclass"/> <xs:element ref="Frame"/> </xs:choice> </xs:group> <xs:element name="Expr"> <! Expr ::= UNITERM > <xs:complexType> <xs:sequence> <xs:group ref="UNITERM"/> </xs:sequence> </xs:complexType> </xs:element> <xs:element name="Const"> <! Const ::= '"' UNICODESTRING '"^^' SYMSPACE  CONSTSHORT > <xs:complexType mixed="true"> <xs:sequence> <xs:group ref="IRIMETA" minOccurs="0" maxOccurs="1"/> </xs:sequence> <xs:attribute name="type" type="xs:anyURI" use="required"/> </xs:complexType> </xs:element> <xs:element name="Name" type="xs:string"> <! Name ::= UNICODESTRING > </xs:element> <xs:element name="Var"> <! Var ::= '?' UNICODESTRING > <xs:complexType mixed="true"> <xs:sequence> <xs:group ref="IRIMETA" minOccurs="0" maxOccurs="1"/> </xs:sequence> </xs:complexType> </xs:element> <xs:group name="IRIMETA"> <! IRIMETA ::= '(*' IRICONST? (Frame  'And' '(' Frame* ')')? '*)' > <xs:sequence> <xs:element ref="id" minOccurs="0" maxOccurs="1"/> <xs:element ref="meta" minOccurs="0" maxOccurs="1"/> </xs:sequence> </xs:group> <xs:element name="id"> <xs:complexType> <xs:sequence> <xs:element name="Const" type="IRICONST.type"/> <! type="&rif;iri" > </xs:sequence> </xs:complexType> </xs:element> <xs:element name="meta"> <xs:complexType> <xs:choice> <xs:element ref="Frame"/> <xs:element name="And" type="Andmeta.type"/> </xs:choice> </xs:complexType> </xs:element> <xs:complexType name="Andmeta.type"> <! sensitive to meta (Frame) context> <xs:sequence> <xs:element name="formula" type="formulameta.type" minOccurs="0" maxOccurs="unbounded"/> </xs:sequence> </xs:complexType> <xs:complexType name="formulameta.type"> <! sensitive to meta (Frame) context> <xs:sequence> <xs:element ref="Frame"/> </xs:sequence> </xs:complexType> <xs:complexType name="IRICONST.type" mixed="true"> <! sensitive to location/id context> <xs:sequence/> <xs:attribute name="type" type="xs:anyURI" use="required" fixed="http://www.w3.org/2007/rif#iri"/> </xs:complexType> </xs:schema>
7.2 Skyline Schema Module
<?xml version="1.0" encoding="UTF8"?> <xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" xmlns="http://www.w3.org/2007/rif#" targetNamespace="http://www.w3.org/2007/rif#" elementFormDefault="qualified" version="Id: FLDSkyline.xsd, v. 0.96, 20080716, hboley/dhirtle"> <xs:annotation> <xs:documentation> This is the Skyline schema module of FLD. It is split off from the Baseline schema for modularity. The Skyline XML schema is based on the following EBNF (which adds Group and Document, and brings 'Implies' into FORMULA): Document ::= IRIMETA? 'Document' '(' Dialect? Base? Prefix* Import* Group? ')' Dialect ::= 'Dialect' '(' Name ')' Base ::= 'Base' '(' IRI ')' Prefix ::= 'Prefix' '(' Name IRI ')' Import ::= IRIMETA? 'Import' '(' IRICONST PROFILE? ')' Group ::= IRIMETA? 'Group' '(' (FORMULA  Group)* ')' Implies ::= IRIMETA? FORMULA ':' FORMULA FORMULA ::= IRIMETA? 'And' '(' FORMULA* ')'  IRIMETA? 'Or' '(' FORMULA* ')'  Implies  IRIMETA? 'Exists' Var* '(' FORMULA ')'  IRIMETA? 'Forall' Var* '(' FORMULA ')'  IRIMETA? 'Neg' FORMULA  IRIMETA? 'Naf' FORMULA  FORM PROFILE ::= TERM Note that this is an extension of the syntax for the Baseline schema (FLDBaseline.xsd). </xs:documentation> </xs:annotation> <! The Skyline schema includes the Baseline schema from the same directory > <xs:include schemaLocation="FLDBaseline.xsd"/> <! The Skyline schema extends, with Implies, the FORMULA group of the Baseline schema > <xs:redefine schemaLocation="FLDBaseline.xsd"> <! FORMULA ::= IRIMETA? 'And' '(' FORMULA* ')'  IRIMETA? 'Or' '(' FORMULA* ')'  Implies  IRIMETA? 'Exists' Var* '(' FORMULA ')'  IRIMETA? 'Forall' Var* '(' FORMULA ')'  IRIMETA? 'Neg' FORMULA  IRIMETA? 'Naf' FORMULA  FORM > <xs:group name="FORMULA"> <xs:choice> <xs:group ref="FORMULA"/> <xs:element ref="Implies"/> </xs:choice> </xs:group> </xs:redefine> <xs:element name="Document"> <! Document ::= IRIMETA? 'Document' '(' Dialect? Base? Prefix* Import* Group? ')' > <xs:complexType> <xs:sequence> <xs:group ref="IRIMETA" minOccurs="0" maxOccurs="1"/> <xs:element ref="directive" minOccurs="0" maxOccurs="unbounded"/> <xs:element ref="payload" minOccurs="0" maxOccurs="1"/> </xs:sequence> <xs:attribute name="dialect" type="xs:string"/> </xs:complexType> </xs:element> <xs:element name="directive"> <! Base and Prefix represented directly in XML > <xs:complexType> <xs:sequence> <xs:element ref="Import"/> </xs:sequence> </xs:complexType> </xs:element> <xs:element name="payload"> <xs:complexType> <xs:sequence> <xs:element ref="Group"/> </xs:sequence> </xs:complexType> </xs:element> <xs:element name="Import"> <! Import ::= IRIMETA? 'Import' '(' IRICONST PROFILE? ')' > <xs:complexType> <xs:sequence> <xs:group ref="IRIMETA" minOccurs="0" maxOccurs="1"/> <xs:element ref="location"/> <xs:element ref="profile" minOccurs="0" maxOccurs="1"/> </xs:sequence> </xs:complexType> </xs:element> <xs:element name="location"> <xs:complexType> <xs:sequence> <xs:element name="Const" type="IRICONST.type"/> <! type="&rif;iri" > </xs:sequence> </xs:complexType> </xs:element> <xs:element name="profile"> <xs:complexType> <xs:sequence> <xs:group ref="TERM"/> </xs:sequence> </xs:complexType> </xs:element> <xs:element name="Group"> <! Group ::= IRIMETA? 'Group' '(' (FORMULA  Group)* ')' > <xs:complexType> <xs:sequence> <xs:group ref="IRIMETA" minOccurs="0" maxOccurs="1"/> <xs:element ref="sentence" minOccurs="0" maxOccurs="unbounded"/> </xs:sequence> </xs:complexType> </xs:element> <xs:element name="sentence"> <xs:complexType> <xs:choice> <xs:group ref="FORMULA"/> <xs:element ref="Group"/> </xs:choice> </xs:complexType> </xs:element> <xs:element name="Implies"> <! Implies ::= IRIMETA? FORMULA ':' FORMULA > <xs:complexType> <xs:sequence> <xs:group ref="IRIMETA" minOccurs="0" maxOccurs="1"/> <xs:element ref="if"/> <xs:element ref="then"/> </xs:sequence> </xs:complexType> </xs:element> <xs:element name="if"> <xs:complexType> <xs:sequence> <xs:group ref="FORMULA"/> </xs:sequence> </xs:complexType> </xs:element> <xs:element name="then"> <xs:complexType> <xs:sequence> <xs:group ref="FORMULA"/> </xs:sequence> </xs:complexType> </xs:element> </xs:schema>